[edent]

The A usually refers to "aisle" in my experience with UK trains. As distinct from a window seat. I think it's like a checksum :-)

[tialaramex]

Nope. Both aisle and window seats will be suffixed A. I almost invariably book window seats, they have an A suffix anyway.

There are other suffixes, but they relate to the layout of seating in a carriage and apparently almost all trains have "airline" style according to the nomenclature of whoever decides these things.