Depends on the company and the times of travel. Southwest trains are usually okay except there can be a few too many bikes so the journey is spent juggling bikes. Dan's Genesis doesn't fit into the bike space so causes a bit of a jam.
If there are disruptions and the trains have replacement buses, you're screwed. No bikes, prams or large items of luggage. Check for engineering works on your planned route. Delays, cancellations and oh, this is modern public transport.

This thread popping up again reminded me of this...

[Future] Scotrail passengers might be annoyed to find that Transport Scotland/The Scottish Government are cutting bike spaces on our new trains from 6 to 2 (plus 2 more if there are no disabled passengers!) on the West Highland line, and from 4 to 2 (plus 2 more in a "flexible space") on the Edinburgh to Glasgow line. http://www.spokes.org.uk/2016/03/cuts-c ... ke-spaces/

If any of you are scunnered enough to try & do something about this then people that live up here can email/write to their shiny new MSPs, and those of you outwith Scotland can email/write to the Scottish Transport minister (I'm not sure if they've been appointed yet).
You could hassle Abelio/Scotrail too, but that probably won't achieve much since it's a government order.

Used a couple of trains when I did the trans Canbrian

Shrews to Knighton wasn't a problem : I can remember booking
Dovey to Shrews : Sprinter style train already had more bikes than capacity so had to wait a few minutes for the next one

Poor train go-ers had a good aroma as the muck on the bike dried out