Hi, I can only speak from my experience but I don't know many PMA's who stayed PMA, unless they were simply were happy in stay in role (for example some I know had child commitments and it suited them to the ground to remain as a PMA and fair play to them - they put family first!) most progressed on to Scheme Project Managers, then Project Managers or other roles such as Sponsors so its a natural step to follow and very very common. All I would say it a lot of Band 5 and Band 4 roles are been abolished more so on the CAPEX delivery front esp on Eastern (read Project ACE on Connect). So I would suspect a bit more competition internally to make such steps. That said there is quite a lot of movement in project teams as people move/progress/leave/project portfolio's expand so a good turn over to create opportunities. A lot of PMA's work out of grade (me included at one point) ie running and leading actual projects so many are actually unpaid SPM's on a PMA wage so even as a PMA you soon tick the JD boxes for a SPM role (if like I say you have a good line manager who encourages you and opens those doors - many will as it means less work for them running the low value projects that don't make it on to Connect).
Don't rule out taking that better paid role in Maintenance (locking yourself into a decent pay range) I know plenty of ex-maintenance folks who move into project engineering or project management role (some failed, many progressed) I've also known Work Planners move straight into PM jobs. So I don't think there is anything wrong as part of your 12 - 24 month development plan saying your long term aim is to move into projects... and see if you can get your department to put you on HSEA courses and APM stuff. Don’t forget many Route Teams have Works Delivery Units that run smaller projects so even a role in maintenance could yield a way into project without leaving the depot.
Without knowing you personally, I would say limit yourself to PMA roles in your job hunt, (considering going for both as they crop up) as above, the boundary between PMA and SPM is grey. One is more supporting, one is more managing, chances are in your own personal life you have managed a project (holiday with the mates, car refurbishment, organising someone’s stag/hen party, painted your spare room) all to a budget and timescale so you can bring all that into any CV or interview and low and behold you have the skills of a SPM gained from your own life. Many SPM’s have been re-badged into new names, but its more or less the same role (usually Band 4c but a few Band 4b).
Sorry its a bit of waffle and perhaps not directly answered your question, but to be honest there is no straight reply.