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The U.K's Doctor Yellows. Network Rail test trains


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Editing my latest Cumbrian Coast video the other day which has a few appearances by the Network rail Infrastructure monitoring trains I started to think about the previous times I have caught these in action. Finding them wasn't easy with my filing system as I've described them in different ways over the years and the old 'Ctrl F' button on Excel didn't find them all.


However there was enough to make a video worth while starting with Network rail's own 'Doctor Yellow' a complete HST set converted for the purpose


Seen here at Tapton Junction back in 2010. The power cars were the so-called Surrogate ones fitted with conventional drawgear and buffers




In 2012 I was waiting at Kirkby Stephen on the Settle and Carlisle line waiting for the double headed Black 5s on the Winter Cumbrian Mountain express steam excursion. Snow still lay on the hills and an icy fog was lifting slowly just in time for the test train to appear.




the other sets either use two diesels 'Top and tailed' or one diesel with a DBSO (Driving brake standard open) control car at the other end. In this case two of our Class 37/4s do the honours climbing Lindal Bank on the Furness line




South of the Settle and Carlisle on the Midland railway's Skipton to Leeds line 37 025 blasts through Cononley with a DBSO at the other end.




One of the coaches sounded like it had some serious flatspots on the wheels!





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Video can be found at



The video that set me off on this is a selection of interesting trains that have passed recently including the 8F 2-8-0 48151 on a steam excursion and tampers and ballast trains






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Just to fill in a bit more detail on the buffer-fitted power cars for anyone who doesn't know the background...


When BR's new Class 91 electric locos were delivered there was a shortage of matching MKIV coaches and driving trailers. In the short term a few HST sets were adapted. Buffers were added to one end coach (HST stock normally using knuckle couplers and Pullman-style gangways in place of side buffers) and the cab end of the power car, which had some extra circuits added to allow it to control the Class 91 at the other end of the train.


As HST MKIIIs use 3 phase ETH supply the power car had to run its engine to keep the lights on. Unfortunately letting the engine idle for prolonged periods caused carbon buildup in the exhaust, followed by fires. The solution? Use the engine for traction power as well as ETH, as there would be no carbon buildup with it working hard. Cue some seriously rapid acceleration from station stops with both a HST power car (which could move the coaches solo at a reduced speed) and a 91 intended to handle ten coaches alone combining their efforts.


After enough MKIVs had been delivered to make up complete sets the HSTs regained their second power cars and went back to their original duties, still bearing the buffers and now-dormant extra control circuits. The buffers made recovering a broken HST far simpler with no need to resort to opening the nose panel and installing the rigid coupling bar. The extra control gear reportedly threw up the occasional odd instrument reading - not dangerous but leading to a few "huh?" moments and much poking around to find out why! This may be why they were passed to Network Rail...

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