If I may be so bold as to suggest buying the Class 153 + Totham, from just trains. The Totham route is amazing and was created by one of the members of this forum. Also take a look at dpsimulation for free add-ons and skins for your 156 (the Cresston route from there is a good one). and you probably know of the delights that uktrainsim has to offer, they have some great freeware stock packs.
Its a brilliant game, I havent got the specs to run TSX mode without serious lag but its just as good with TSX switched off, in fact when driving a Deltic in the rain, TSX off is best because the wipers hardly clear the windscreen.