And if it’s on its way through Llansamlet enroute to Swansea when the Up Sleeper fails at Camborne? Your special unit is 300mi away, so you’re left subbing a normal IET despite all the expense on the “special” replacement. So you’d actually need a whole fleet of them, and diagram them (and keep them on those diagrams) so that 1 was always near-ish Paddington with enough Fuel to get to Penzance at 2300 each evening, and 1 was always near-ish Penzance with enough Fuel to get to London at 2100 each evening.
The existing contingency measures are well practiced, and well-enough received by passengers. On top of that we’re generally talking 2-3 times a year tops that it happens.