Is the role directly employed by the train company, or as is often the case with cleaners is it for an external company supplying cleaners? (Often on much poorer conditions then if they were directly employed). If the latter, I'm not sure how useful it be for onward progression.
Also, whilst not wanting to denigrate the great job done by our cleaners, I see no reason with your background why you couldn't apply for jobs traditionally considered to be further up the chain, like conductor or station staff, if you don't feel ready yet to apply for a drivers job?
For the record, I worked up from catering staff to platforms to guard to driver, something I'm rather proud of, but there were still more external trainees completely new to the railway on my driver's course then internal (in fact there were on all 4 training courses I've done for each role) so whilst being internal can help, and it will definitely help supply you with lots of useful experience that you can mention in job applications, it's by no means a requirement.