I had idea years ago regarding the problem of increased demand on some routes in the summer (for example Swansea/Carmarthen to Tenby). Ideally, what you need is some trains that sit around most of the year and come out only at the busier times, but it's not economic to do that - you need to have the fleet earning it's keep all year round. And then I realised that there are trains (and indeed entire routes) which sit around not doing much at times of lower demand, in the form of heritage railways.
Unfortunately, the issues of low speed limits etc. on heritage railways, PRM regulations etc. are almost insummountable so this will probably never happen in reality but the sort of heritage crossed with national rail operation on the Lymington branch when the last couple of 3CIG units were working it gave a slight glimmer of hope that it might not be completely impossible. What would be needed is a route with a basic Public Service Obligation (PSO) timetable (let's say a train every two hours) that runs throughout the year. In the quiter periods, this would be operated using normal 'modern' stock. At busier times, additional services (worked by heritage stock, such as the 3CIGs in the Lymington example or a heritage DMU (such as the class 121s that Chiltern and ATW used at one point, or even steam) could be run, (making the service up to hourly in the example). Finally, and this is key, when peak season is reached the PSO services would also be operated using heritage stock, freeing up the 'modern' stock to strengthen other national rail services. You could also run just the PSO services (ie. only every 2 hours in our example) with heritage stock at times when the national rail network needs the 'modern' stock but demand isn't sufficient to justify the full hourly heritage service.
