I think the 156s and 158s are very unsuitable for the, Fort William, Kyle Of Lochalsh, Mallaig, Oban, Thurso, Wick, services. It is a joke that such stock is used on what are some of the most scenic train routes in the country. I think the 170s and 185s would be perfect for these lines.
Also the 153s are a very poor choice for the Heart Of Wales line services. Again this line really needs some higher quality stock like the 170s and 185s as it is a very scenic line.
I would also say that the 345s are very unsuitable for the Crossrail services. The class 700s are what should have been ordered. They are similarly high capacity with very wide spacious aisles but instead they have normal (not longitudal) seating and are a much better design and far more pleasant to travel on.
Finally the 158s and 159s on the Salisbury to Romsey stoppers are not very suitable. SWT used to have 170s which were perfect for this work but the stupidly got rid of them.