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In the BR era ( 1970/80 ) HST's running on 1 power car were common when the Paxman engines had overheating problems. Was on several where it was a slow climb up from Stroud to Sapperton summit.
They would start the diesel engine a few minutes before.
Then at the end of the 3rd Rail, close the electric controller and open the diesel controller.
For "Worst Railway Journeys in the UK" my vote would be a journey from Cardiff to Manchester in a TFW Class 153.
There were several today according to Realtimetrains.
A bit slow but there is a bus No 103 between Colchester/Manningtree/Harwich
Suggestion:
Get the train from Liverpool Street to Manningtree
Walk to the Skinners Arms ( About 10 mins )
Then 103 bus to Harwich
I was there on BR in the 70's/80's
The scenario was just the same with restrictions on who does what and where.
Co-operation between regions for Crews/Locos/stock etc could take several days if it happened at all.
I am afraid BR is not a good example to plan for the future.