The title says it all, I've asked that question since Grand Central came onto the rails
Their small 3 train fleet is all buffer fitted Class 43 locomotives with a rake of hauled-type British Rail Mark 3 carriages which had a full rewire to work with Class 43 (HST), although the BREL designed interior of these carriages was retained with the exception of game boards on the tables, that's a Grand Central thing, quite clever if you ask me :)
The InterCity 125 trains in use is in a distinctive all black livery with a small yellow panel between the lights and windscreen, all locomotives and multiple units on UK railways require a yellow front, Grand Central seem to be the worst ones adhering to that
The fleet have suffered reliability problems, but these have been about ironed out now
I will be taking a trip on a Grand Central train in the near future, I'll post from there and let you know how the service is :)
more soon