Ive been on a 2 car 175 train for Manchester to Cardiff a number of times. Really the route should always be a 3 car operation though obviously there arent enough of them. First Class I think would be quite popular also, as it isnt just Manchester and Cardiff but, Newport, Hereford, Shrewsbury and Crewe which are all big towns/ small cities.
One time it was awful, it was the 1.30pm from Manchester on a hot August day. Leaveing Manchester it was moderatly busy, but quite a few spare seats. From then it just got busier and busier, in Crewe a big pile of people got on, then at Shrewsbury though quite a few people got off even more got on and there was really no standing romm left never mind seats. Though it would get worse, we got to Hereford at about 3.45 and then about 40 school kids piled on and the train must have benn 40 degrees plus, and I thought I was going to pass out. Then after Abergavenny it got quite a bit quieter as loads of people piled off. It was ok to Cardiff then, but still quite busy.
When you get a 158 on the route it isnt very nice either.