Certainly older trains were never really designed to be switched on and off, and the heating/cooling cycle will reduce the life span of an engine, especially big diesel engines. If this was attempted now with the age of some DMUs you will quickly find failures all over the network and a reduced MPC for the fleet. Modern manufacturing techniques on newer trains have probably alleviated this problem to some extent, but life cycles will still be reduced if they are regularly switched on and off. Although car engines are getting better, older engines built at the same time as Class 14x and 15x series will usually be in need of scrapping or overhaul at 150,000 miles, whereas rail engines can manage a million miles or more, one of the reasons is the many heating and cooling cycles a car goes through.
One partial answer to the problem is to use engines that have the ability to continue operating on on fewer cylinders just to maintain heat and avoid relying on the batteries to fire it back up again.
I was told by an EWS (as was) driver that they were instructed to start turning engines off if they were idling to save fuel, and then wondered why many of their trains were sitting down all over the network. Saving pennies on fuel cost them thousands in delays.