How fast can a Deltic go?

How fast is a Deltic?

by Ian Strange

In cab of Deltic D9009

Some of the famous high speed steam runs of the 1930s, in the UK, stimulated interest, in the way that publicity stunts do, although safety concerns helped to put an end to it. Progress had been made in building locos able to achieve speeds over 100mph, but brakes had not progressed! Tests for improving brakes provided the scenario for Mallard's successful attempt at the steam speed record, running down Stoke Bank with 7 coaches (including dynamometer car), touching 126mph. With this world steam speed record achieved in 1938, there was less incentive for excess high speed attempts. The partial run-down of the British railway system over World War 2 also meant that it would be a few years before high speeds could be considered again.

With the coming of diesel fleets from the late 1950s, there is an important difference to appreciate between steam and diesels at high speed: A steam engine can be nursed gently prior to a strenuous burst of speed, usually down hill. That is handy for record attempts, but of little value for transporting passengers, who require a high average speed. A diesel loco's performance is more predictable and can be maintained continuously. A Deltic has the power to maintain speed up hill, or with heavy loads, and accelerate fast, which provides those rapid journey times.


When it comes to seeing how fast a diesel loco can go, one limiting factor is known as "unloading". In the case of a diesel-electric, the traction-motors generate a back-EMF that opposes the generator's output. The generator fights back by increasing its field strength (or "excitation") as the train speed rises. At maximum excitation, field-diversion cuts in, and the sequence continues as the train accelerates. Once the field-diverts are used up, and excitation of the generator hits maximum again, the power output of the generator begins to fall off as speed rises further, and the engine governor reduces the fuel to the engine to prevent over-revving. This reduces the power available for much higher speeds.

Deltics are slightly different. Apart from their sheer power, they also have 2 generators connected in series providing a high voltage. While most first generation diesels reach their unloading speed below their maximum operating speed, Deltics have to exceed 100mph before they unload.

Traction Motors

When the Prototype Deltic entered service (1955), its traction motors were geared for a nominal 90mph maximum running speed. Subsequently, they were re-geared to permit up to 105mph. Part of Deltic folklore is the tale of a driver getting a tad carried away speeding down Shap. The actual speed was not recorded (as far as I know), but it was sufficient to burst the commutator bindings on all 6 traction-motors, from the centrifugal forces. That highlights one of the limiting factors, as with any form of traction, since centrifugal forces rise with the square of speed. Mallard's 1938 steam record run, caused some damage when one of its big-end bearings failed.

A much more common type of damage, is the flashing over of traction-motors. That is when sparking from the motor-brushes, jumps to the wrong commutator segment and short-circuits the transmission. This was normally caused by a combination of high speed and other factors, e.g. Uneven wear of commutators, or the loco bouncing over junctions, causing the brushes to bounce. Fortunately, English Electric motors were so well built, that damage was not always serious.

Contrary to what some books state (even English Electric publications contain mistakes!), the production Deltics' motors had a different gear-ratio, 53:18 (ref.1). They were geared for 100mph running, but keep in mind that they also had different traction-motors with 4 poles rather than 6.

When the Deltics were rewired around 1969, the protection circuits were improved so that there would be less damage if flash-overs occured. Upgrades to the East Coast Main Line, plus instructions to drivers to ease off over junctions, meant that high speeds were less likely to cause damage from the 1970s onwards. One last factor to remember, is that wheel diameters reduce as they wear and are then turned on a wheel-lathe, thus increasing the traction-motor revs for any given speed of loco travel. The former status-quo is returned when new tyres are fitted to the wheels.

Deltic Power

With the introduction of air-conditioned coaches in 1971, the power available to the locomotive wheels reduced, with about 30hp electrical load taken by each coach.

The version of the Napier Deltic engines installed in the locomotives, were rated at 1650bhp continous (3300 total per loco) at 1500rpm. The Napier sales brochure quotes them as 1750bhp continuous and 1825bhp short-term rating (also at 1500rpm). Other versions of this model of Deltic engine (designated 18-25B) were quoted at various lower and higher figures depending on the application (ships, industrial, etc.). Some tweaking of the controls (on purpose or by mistake!) might bring out a little more than the advertised 3300bhp rating.

Speeds achieved without the assistance of gradients and very light loads, can be a useful demonstration of power. A Deltic can cruise steadily at 105mph hauling 12 coaches on a level run. The world steam speed record (126mph) was set descending Stoke Bank with 7 carriages. Deltics ascending Stoke Bank with 7 carriages could take the summit at up to 102mph.


The production Deltics were officially limited to 100mph. This figure would have taken into account the traction motor mechanical forces and dynamic forces by the wheels against the existing trackwork. The Deltics were particularly good in the latter respect, thanks to a fairly low unsprung mass (and low axle weight). In fact, the "Deltic Criterion" became the train designer's "yardstick" for subsequent fast trains. That is, the forces that Deltics impart to the rails at a joint at 100mph, was set as the maximum acceptible for any new train at whatever its maximum speed was.

One also had to consider braking performance with the coaching stock of the day. With the conventional clasp brakes, a small increase of speed above 100mph, called for a disproportionately large increase of stopping distance.

The carriages themselves would have been restricted to 90mph initially, until Commonwealth and B4 carriage bogies entered the scene, to raise the permitted speed to 100mph. And finally, the speed limits on the East Coast Main Line were a maximum of 90mph, until 1964 when the first 100mph signs were unveiled. By 1978, the line speed limit rose to 125mph for the introduction of the High Speed Trains (HSTs). While Deltics (and the carriages they hauled) were still supposed to be restricted to 100mph, drivers were more inclined to break that limit once the line had been upgraded, especially if running late.

Early fast runs

Railway engineer, the late Irving Wells, rode in the Deltic cabs during their acceptance runs after delivery (ref.2). In the case of number D9011, its speedometer failed as it began the descent of Stoke Bank. Irving was pleased about that and gave the driver instructions to keep the loco wide open. He noted that the traction-motors started to whistle, such was the rate of speed. Eventually, the traction inspector (holding his stop-watch) spoke up and commented that the mile-posts were being passed every 30 seconds and the train was still accelerating! That was 120mph (193kmph) and rising. They couldn't tell anyone of their exploits for many years, otherwise they would have been fired!

Settle and Carlisle Line Record

On 17th June 1967, Deltic D9005 "The Prince of Wales's Own Regiment of Yorkshire", hauled a charter-train, the "Hadrian Flyer" (ref.4), made up of 11 mark 1 coaches. This traversed the Settle to Carlisle line in the southbound direction. The driver seemed to get a touch of the racing driver's "red mist" and set a record that still stands at the time of writing (2017). The maximum line speed limit was 90mph in those days, and this train covered the Carlisle to Skipton stretch at an average speed of 71mph. This line was subsequently down-graded, safe-guarding that record.

Incidentally, there is a brief piece of film footage of this remarkable run in the programme and video "Deltic, King of Diesels", albeit without caption or commentary reference. If you have the video, look for a Deltic in green with full yellow ends, and carriages in a mix of liveries, at approx. 7.5 minutes in.

The Race-Track: York to Darlington

The stretch of line between York and Darlington, provided an ideal venue to see what speeds were possible without the benefit of gradients. Surely, going downhill is cheating?! On this 44.1 miles run, the challenge for Deltics was (unofficially) to break the 30 minute barrier. Interestingly, preserved Deltic D9000, broke that 'barrier' twice in 1998 on service trains for Virgin Trains. Both times, I don't think 102mph was exceeded.

For high maximum speeds, we need to look at occasions when delays prompted drivers to put in a heroic effort. One of the best reported runs (ref.3) occurred in the 1970s when there was a speed restriction in force of only 20mph at Northallerton. Hauling 9 coaches, Deltic 55009 "Alycidon", was already above 100mph, 10 miles out from departing York. Speed settled at between 109 and 113mph for about 8 miles until braking for the temporary restriction. Despite that, arrival at Darlington was 8 seconds late, against the 32.5 minutes schedule.

The Hull Executive

With the first batch of Eastern Region HSTs entering service on the East Coast Main Line in 1978, there were not enough to serve the city of Hull. To keep the Hull commuters happy until more HSTs entered the scene, a stop-gap weekday service was launched: The "Hull Executive". Exclusively Deltic hauled, it was fast. And I mean really fast! Of particular note was the evening return journey which was non-stop from Kings Cross to Retford. The schedule required an average speed, start-to-stop of 91mph. Even with a clear run, it was well nigh impossible to achieve within the 100mph nominal speed limit for the Deltics and the mark 2 coaches. This schedule was tighter than any other locomotive hauled service, diesel or electric. Published logs of some of these runs show that drivers were cruising at up to 110mph to maintain the schedule. After a few months, an extra station stop was added at Newark, and the timing eased to require an 87mph average to that first port of call.

One run that was logged and published (ref.5) featured Deltic no.55005 "The Prince of Wales's Own Regiment of Yorkshire". With the usual 8 coach (271 tons tare) load, the driver faced a temporary speed restriction down to 20mph just south of Peterborough. This prompted some very fast running to maintain schedule, although initially hovering around the 100mph mark, then a surge to 107mph north of Hitchin. A signal check slowed it to 60mph before a surge to 110mph, maintaining 3 figure speeds until that 20mph restriction. A 109mph run at Stoke Bank allowed 3 figures to be maintained up the long climb with 100mph at the summit. From Grantham, speeds were mostly above 100 again, with a maximum of 110mph once more before being signal checked on the approach to the Retford stop. Arrival was almost 1 minute early.

Not So Semi-Fast

55009 passes Huntingdon

Once HSTs were making their mark, most Deltic workings were on so-called "semi-fast" services between Kings Cross and York or Hull, and a few venturing further north and elsewhere. These services called at numerous stations, providing a series of short sprints.

As was often the case, the best fast runs occurred in the face of severe delays and heroic efforts by determined drivers. Possibly the best recorded run was logged and published in the journal of the Deltic Preservation Society "Deltic Deadline" (ref.6). The run took place in 1979, on the 8 coach 10.05 Kings Cross to York. One can almost sense the frustration of the driver, hell-bent on not being defeated by some appalling delays. Setting off 25 minutes late, with an HST only 20 minutes behind, speed rose into 3 figures once passed the initial climb to Potters Bar. 112.5mph was maintained for a few miles until a signal check, after which speed rose to 118mph! Average speed to the first scheduled stop at Huntingdon, was 88mph. On the short sprint to Peterborough, speed hit 114mph. Another severe delay evidently did not impress the driver who then took Stoke Bank at up to 90mph... on the slow line! An amazing sprint from Grantham to Newark saw up to 114mph again, over that short stretch.

The Retirement Run - 55008

The fastest run that I know of (that is believable!), was written up in "Railway World" magazine many years after the event took place back in 1978. Unfortunately I've lost track of the magazine issue and so can't quote the reference. The locomotive was 55008 "The Green Howards", and the 'event' was the retirement of a driver. This was very much an unofficial event! It was carefully planned so that a Deltic was available with excellent running gear, and wheels at maximum diameter. What is more, this was a normal scheduled service train with passengers, 10 coaches load, rather than a light-weight test or empty working.

The service was from Newcastle to Kings Cross and speeds above 110mph were recorded prior to the descent of Stoke Bank. There, the maximum was recorded as 125mph. Whether or not the driver switched off the ETH to gain a little extra power, I don't know! With a lighter load, and if the traction-motors held out, then unloading would probably prevent anything much above 130mph. However, I would not recommend experimenting! Tests of speeds ascending gradients are a more interesting - and safe - challenge.

Heritage runs... or how a museum piece showed a clean pair of heels to the rest!

After D9000 returned to the main line as a preserved loco, it made history by hauling some scheduled service trains from 1997. A journalist in the "Sunday Observer" felt moved to comment that he did not want a "museum piece" hauling his trains. Here's a news-flash for him: Those trains were already hauled by "museum pieces"! Apart from its perfect reliability, Virgin Trains' drivers made the most of this Deltic's power, and set some records in the process. An out-right record time from Coventry to Kensington Olympia. Admittedly, that's not a route to get many other challengers! More impressive is a diesel record for the short start-to-stop sprint from Coventry to Birmingham International, a route traversed countless times by HSTs and class 47s. Also, the fastest loco-hauled time between Exeter and Taunton, reaching 99mph on the uphill section! These were typically only 7 coach trains, but with full ETH load sapping some power. I should really say "ETS" load, since it now included powering the microwave oven in the buffet!

Some other noteworthy fast runs for D9000, included substituting for powerful class 86 electric locos. In November 1997, this Deltic took one of these (normally electric) Virgin services from Birmingham to Glasgow, as an effective test run to raise D9000's mainline certificate from 90 to 100mph. The usual 7 coach rake was lengthened to 8, yet delays enroute were largely clawed back, despite the challenging schedule. The next day, it returned on an Edinburgh to Birmingham service. I won't discuss maximum speeds, since senior eye-brows were raised at the time (!), but there was a splendid display of rapid standing starts from station stops, witnessed by young and old enthusiasts alike. Arrival back was on time, albeit with some recovery time in the very last leg (it was 1 minute late departing Stafford when I saw it).

In another demonstration of uphill speed, during a cab-ride I had in D9009 "Alycidon" in 2002, we made a standing start from Tallington and tackled Stoke Bank on the fast line. Our trailing load was 12 mark 1 coaches and we attained 78mph, dropping back to 75mph at Stoke Summit. This was logged by a friend on-board the train.

Licking the Lickey

At 1 in 37.75, and about 2 miles long, the Lickey Incline is a classic challenge. The speed limit on the approach is 75mph. The first time a Deltic took on the challenge, was on one of the final railtours in BR ownership. 55022 hauled the "Deltic Venturer" tour on 28th November 1981, and reached the Lickey after nightfall. On the day, there was a temporary speed restriction of just 20mph at Bromsgrove, and the trailing load was 11 mark 1 coaches. Once clear of the restriction, the Deltic accelerated from 21mph to 30mph at the summit (refs.7 and 8).

The next assault by a Deltic came in 1997 with the same Deltic, under its earlier guise of D9000. There was a temporary restriction at the start again, but 50mph this time. With 12 heavy coaches (mark 1s with Commonwealth bogies), speed fell to 29mph at the summit (as logged by a friend of mine). I estimated that a balancing speed would have been 26mph, assuming the full advertised horsepower was available. On a subsequent run for Virgin Trains, with a much lighter 7 coach load, the full 75mph speed limit was back in force, and speed fell to 56mph at the summit, including a rare field reversion (field divert cutting out).

For comparison, here are some logged runs by other classes of loco (ref.8):

Class 31 with 10 coaches and banked by a pair of class 37s, started from a stand, took the summit at 32mph.

Class 45 with 12 coaches, started climb at 77mph, took the summit at 25mph.

Class 47 with 11 coaches, started the climb at 62mph, took the summit at 28mph.

HST set (7 coaches) started climb at 78mph, took summit at 60mph.

5MT steam engine, banked by a pair of 3Fs, with 12 coaches, started from a stand, took summit 14mph.

A fews weeks after D9000's 1997 ascent, a class 50 railtour with load 12 took the summit at 23mph. I don't know what its starting speed was? (Can't find my old reference for this run).

To sum up...

In practice, Deltic performance far exceeded what the simple "100mph" label might have suggested. So, even though an outright maximum speed attempt would probably only improve marginally on the A4's steam record, a very high maximum was not needed for it to do its job. As the HSTs demonstrated, consistently higher speeds required a redesign: Brakes, transmission, axle-weight saving, etc. for the coaches as well as the motive power.


1 - British Transport Commission 3,300 H.P. Diesel Electric Locomotives Type 5, Book No.170 (E. E. Co. Ltd). I've also counted the teeth on a pinion gear, at 18.

2 - "Deltics 1955-78". VHS video published by "Transport Video Publishing".

3 - "Railway Magazine" July 1978, Deltic run logged by Mr. David Foy.

4 - "The Heyday of the Deltics". Photo book by Gavin Morrison.

5 - "Railway Magazine" February 1980, Hull Executive run logged by Mr. David Fox.

6 - "Deltic Deadline" June/July 1996, Deltic run logged by Christopher J. Oldham.

7 - "Railway World" magazine, April 1982, write up of Deltic Venturer tour by P. A. Rutter and C. J. Lejeune.

8 - "Railway Magazine" July 1982, Locomotive Practice and Performance, by P. W. B. Semmens.

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