Is there not an ETH converter vehicle that could be added to the sleeper set so that the Class 43 power cars could be added at either ends of the train rather than back to back?
There was a mark 1 generator vehicle converted for use on the Western Region to provide the three-phase train supply to HST mark 3 rakes in the late 70s/early 80s when BR operated a loco hauled HST mark 3 rake due to a lack of serviceable power cars. It has since been bought by Riviera Trains, and has been converted again so that it now provides standard ETS power to mark 1s and mark 2s. So that could be used to provide ETS power to loco hauled mark 3s, although it doesn't convert the supply from the power cars, but instead provides it's own power source.
Sounds like an awful lot of trouble to go to mind, and as has been outlined it would not be possible to operate the HST power cars in multiple due to the difference in their control systems. It's incredibly noisy too - You wouldn't want it to be coupled adjacent to the sleeping cars!
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Another variation would be to equip the sleepers with DVTs with generators as Chiltern do. FGW seem to propel at Penzance as a regular movement so this could be beneficial. For Scotrail replacing the final brake in the consist with a DVT could allow them to propel into Euston, so saving a loco movement and replacing a Mark 2 and a 90 with a DVT, which ought to assist with the platform length issues should other Mark 2s be replaced with Mark 3s.
I don't think that you would even have to remove a mark 2 from the Caledonian Sleeper in order to fit a DVT onto the rake, as the DVTs aren't much longer than the 90 that they would replace at one end of the train. Using a couple of DVTs on the sleepers could be a smart move. It would save a number of complex shunting and light engine moves at Euston, not least the movement of a class 90 across the station throat in the middle of the morning peak, having been released from the stock of one sleeper to then attach onto the country end of the second rake to take it out to Willesden.
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Doesn't TDM use normal RCH jumpers (I'm thinking about the GA 90s+Mk3s)? So the Mk3 sleepers should be already capable of being married up with a DVT or it shouldn't be hard to modify them at any rate. As for being scrapped the Night Rivera is safe for at least a little while yet as it's being specified as part of the Greater Western franchise.
You are absolutely right, the TDM system uses the standard RCH jumpers, so all of the loco hauled mark 3 vehicles, including the sleeper cars, are perfectly capable of working in push-pull mode automatically with any TDM fitted loco, no modifcation needed.
It is interesting to note that during the testing of the class 91s on the ECML when they were brand new, many of these test runs were formed up of redundant mark 3 sleepers, which worked in push-pull mode with a class 91 at one end and one of the converted "surrogate DVT" HST power cars that had been fitted with TDM equipment at the other.
Check out this image for proof:
http://www.traintesting.com/images/43013 on test train Doncaster to BN 12-05-89.jpg