Just to clarify, is this aimed very heavily at enthusiasts, or is it also for more "experienced" passengers who may want extra timetable detail, station info, etc., without necessarily having a real interest in the railways?
Aimed at entusiasts
If enthusiasts only... here are a couple of pointers. I don't mean to sound stereotypical, but sometimes drawing up an imaginary "persona" of a potential generic user can be really useful.
Is it aimed at any older or more traditional enthusiasts who might want to mimic "analogue", real-life methods, or would the affordance (how much something is recognisable compared to real-life situations/objects, e.g. notebooks) be able to be more limited?
Is it aimed at any children who might be interested in the kids' side of a heritage line but might not have any passion whatsoever for a load of modern multiple units?
Most probably not, as I want this one to be a spotters/enthusiasts one, but maybe a second app in the picture...
As for the in-betweens and further details of the above two paragraphs' descriptions... who are they? What do they do? Are they just spotters, or photographers, or interested in operations (e.g. timetabling)? Do they like to stay in one place all day, or do they like to "bash" lines or move around a bit? Or would you like a whole spectrum?
Maybe a spectrum, it would be difficult to encompass as much as possible, but that's what Id like to try
There are a huge number of different types and sources of data, features, ideas... and ways you can build an app. You will really need to know which group of people you're aiming to target - and know them well!
People like the ones on here as they are the ones that called for an app. But as said, others could always be in the pipeline.
Start with maybe a recognition of locos/units, a checklist of all the numbers and some maps and TOC info perhaps? Other bits could be added in updates?
Thanks 
