I don't think the edit about being at college some days a week was there when I responded before. That's going to rule out full time work, and what with the existing (legal) limitations on safety critical work and shifts, may rule most things out for now.
I'd say it's certainly worth making contact with local TOCs and asking whether they do (operating) apprenticeship schemes and if so when they recruit, what the age ranges are and so on. The FCC scheme seemed to imply that they would recruit a group of people in one exercise, rather than in ones and twos through the year. From memory I think the scheme was open to the 16-19 age group. (I wasn't looking for me, as I passed this age longer ago than I care to admit, but let's say I remember the jam first time round...)
Making contact to ask certainly won't do you any harm.
I'm not best placed to advise, but it will probably be better for your long term prospects to stick with the college course you're doing and then look for a job when the course is finished, rather than abandon the course now and look for the sort of job you can get at 16.
With saying you're available weekends, one thought might be the sort of 'auxiliary' role like looking after rail replacement buses which tends to be mainly a weekend job. First Rail Support has a team of people who do weekends as and when required, and they look after the Northern Rail area. More
here - their blurb doesn't specifically say you have to be over 18, but there may be the same issue of shift times, and there's also the practical consideration of being able to get to and from stations where there is work (bear in mind that a duty might mean being there in time to despatch the first departure, or staying to see the last arrival in, so don't depend on travelling on the rail replacement service yourself). Again, an enquiry isn't going to do you any harm.
I would agree with E&W Lucas about making sure you don't come across as thinking you know all about the job already.
Also (and again I'm hoping someone else may be able to comment here) some years ago, there did seem to be an official attitude that they didn't want "train spotters" going for jobs on the railways, so it was advisable to play down any enthusiasm, even if you were a responsible sort of person with an interest in railways rather than a "train spotter" type.
A lot of job interviews include "give an example of a time you have done X" rather than "what would you do if X" type questions (competency based interviews) so voluntary work of any sort will be an advantage for a young person who hasn't got much employment experience to draw on.
The fact that you will have experience of working with the public (for questions like "give an example of a time you gave good customer service / a time that you dealt with a difficult customer"), and working in an environment where there are safety rules and regulations to follow, and where you have to know how to deal with an emergency, will all be good.
