If it's the UK, then you can book a seat on most services... if you're travelling distance during peak time, and you know in advance, then I'd recommend it - and being prepared to politely but assertively ask people to move if they are sat in your seat.
One thing that comprehensively confuses people using UK railway train reservations: The 'A' suffix on your seat number doesn't matter. When you have a reservation for 44A that doesn't actually mean there is a seat 44A distinct from seat 44, it's because somebody thought it's important to bake in metadata about how seats are arranged in trains, the A apparently stands for "airline" style for some reason.
If you hate reservations or are too disorganised to book one for a busy train, but also dislike being disturbed by someone who does have a reservation and wants you to move, most train operators have some rule about which parts of their trains aren't reserved, you can find that out and then travel in the unreservable part of the train. Finding a seat there won't be any easier, but you won't be asked to move later.
It is interesting how trains are offering better services even as planes suck more over time. Ten years ago knowing where to find power sockets in the Standard class 444 series (next to the cab at the far end from First class) made the difference between charging devices and going without, now almost every train has power sockets and/or USB, free WiFi (not _good_ WiFi, you won't be streaming video - but it'll let you check email and read Hacker News) and air conditioning.
> Finding a seat there won't be any easier, but you won't be asked to move later.
It used to be really easy. Reserved seats had a paper ticket stuck in the back. Virgin trains switched to a small LED display on the edge of the luggage rack. Rather than have a single word like "Vacant" it has a scrolling message. As do reserved seats. So you have to peer at every single display as the text slowly scrolls past to figure out where you can sit. I don't know if this is gross incompetence or part of their ongoing efforts to discourage unbooked travel.
The longer scrolling message is telling you, as you'd know if you read it, that this seat can be reserved even though it currently isn't. As I indicated, some seats can't be reserved, and in this case the message is far shorter and doesn't scroll. "Unreserved" is usually the word used.
Paper tickets can't be updated and so in some cases they reflect no longer accurate information. This is especially misleading for long distance services where many reservations may happen after the service departs from its origin.
The unreserved message definitely used to scroll without any mention of future reservations. Perhaps they've updated them or perhaps it varies by region.
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.
