Small (amber/yellow
LED matrix displays at small intervals (5m? 10m?) along each platform, with the coach number (and ideally seat numbers in that coach for the nearest door, and in fact an arrow pointing to the location of that door or the nearest two doors to that sign, with the coach and seat numbers for those two doors shown) which will stop next to that sign, for the next train serving that platform. Those signs which are beyond each end of where the train will be, for any particular train, show arrows pointing to where it
will be.
Should/might be possible to get away with solar panel on top to charge a few 18650s inside each sign (avoiding expensive attachment to mains), and connectivity via Bluetooth LE (facilitated by each sign participating in a mesh network, updating being relatively infrequent, …) with one master per station with a WiFi or LTE uplink. Could reduce power needs further with a PIR motion sensor and light sensor to light it up only when someone's nearby. White downlighter “puddle” LEDs (when batteries in a good enough state of charge) also thus activated would be a nice touch. Using an LCD or e-Paper display (using LED or electroluminescent front or back lighting) would reduce power needs further. For configuration, it should even be possible to self-configure for distances once one sensor per platform knows “where it is” (most obvioust way to achieve that being to put the “front” indicator of a given set onto the stop-board, or otherwise a non-indicator beacon for the others to reference at that stop board. After that it's a pretty straightforward software problem, but then again, isn't everything?
(ahem, GWR IET seat reservations
) …