One of the reasons I stopped commuting by train + bike was because the route on which I travelled is incredibly busy. I use the Robin Hood Line, which is Worksop/Mansfield to Nottingham and got on the last-but-one stop, got off at Nottingham, then got on another train to Attenborough or Beeston and cycled from there. The Robin Hood Line was always busy, the Attenborough/Beeston section was always problem-free.
The mornings were usually fine, occasionally you'd get more than the permitted two bikes on, but the evenings were horrendous. On one occasion there were SIX bikes, a scooter, two prams + suitcases all in one space. Passengers always stand on this service (1726 ex Nottingham), the next one half an hour later isn't so bad.
On that service, operated by East Midlands Trains, I would try to wait halfway along the platform so I could watch the train pull in and look for the labels by the doors. If the wheelchair label was at one end, that meant the bike one was at the other, but they are very hard to spot and sometimes are obscured by dirt.
If getting on at Beeston, the CrossCountry services usually have a pretty decent bike area. The East Midlands Trains Meridians have it at one end, usually the back, and they're a bit like an old guard's carriage.
My commute consisted of a 2-mile cycle at either end and 2 trains in the middle. I only did it for a short while and in the end it was part of the reason I quit my job, which really was too far to commute without a car or without it being prohibitively expensive (for my means anyway). The other cyclists I met were usually very helpful and understanding but occasionally you would get one who was a bit of a knob (for example people who would insist on bringing a bike onto a train that was already packed and already had 3 bikes on it). Passengers too were largely understanding but sometimes just cheesed off. One cyclist told me a man refused to move from the fold-down seat in the cycle space, even when the guard spoke to him, and he ended up getting a later train. I have on a couple of occasions decided to wait for a later train.
The problem I found really is that train carriages (on the Sprinters) and doors are really narrow, so if you get on with a bike and find there's no room, it's hard to turn it round or lift it up and get it off again. Not to mention that unless your train is starting from your starting point, you have a matter of seconds to get on, and to be honest if I've got on with a bike and found the space occupied I haven't usually felt able to get back off again.
I would never commute again with a bike on the Robin Hood Line, it has really put me off. I would use it for leisure purposes but not regularly, there is too much stress and risk (of not being able to get on) involved.
I do think cyclists should be allowed on trains but provision needs to be made better. I would like there to be more spaces but I don't really know how. East Midlands Trains doesn't have compulsory reservations on my local routes but maybe if all TOCs did have that it would be easier to work out routes which carry a high number of bikes and start there, looking at how it can be done.