I don't work in train planning, but with that caveat -
As has already been asked, is it a secondment? Generally, if an organisation offers a temporary role to internal candidates, it's on the basis of it being a secondment, in that you go back to your previous job at the end of it rather than have to apply for jobs or leave.
Having said that, I'd be surprised if a TOC would invest 15 months' training for a role like this if it didn't intend (even if it's not offering a guarantee) to keep those people on in the new role, I can't see any TOC running a scheme like this so that individuals can go and get jobs elsewhere at the end of it.
Train planning is something of a niche - there's not a huge number of vacancies, but I don't think there's a huge number of experienced train planners out there looking for jobs.
I'm aware that some TOCs have advertised for trainee (i.e. who have not done a scheme like this) as well as experienced train planners in the last year or two, so as with any job, new people are brought in from time to time as well as experienced people - there's going to be some experienced train planners who for one reason or another are willing to move to a different TOC, but probably not enough to fill all the vacancies.
Similarly, Network Rail take on trainee timetable planners from outside the industry now and then.
I have seriously considered both in the past year and for personal reasons not pursued it. The roles I'd considered were on the basis of full time employee status rather than apprentice status, although with any new job there's a trial period and broadly speaking you've few rights even to try and claim unfair dismissal within the first 2 years of any job.
Trainee salary (London and south east) seems to be around the 25 K range, experienced jobs start around 30 K.
Obviously you know what you're on now and how long term secure your current job is likely to be (predicting job security is not an exact science at the moment) but there's going to need to be some train planning function needed in the future even if that's mainly to plan cuts in the short term.