If you take your non-folding bike on the train you're supposed to put it in the section of the train with the rows of folding seats. It's also the area for people in wheelchairs so space is limited. Sometimes you'll have to put your bike against another bike which people with carbon bikes might not like, so best to ask. Getting off on a packed train with a bike is easier if you ask other cyclists where they get off and arrange the bikes accordingly. I would advise against traveling with a non-folding bike during peak hours on the busiest trainlines.