I can't imagine it would be that hard for realtimetrains (or a similar site)
...
Surely somebody is working on this? All it would require is to maintain an archive of past real-time running data.
People on here seem worryingly keen to put me out of a job?!

hock:
Seriously, while what you describe might work for a few WTT paths,
the lack of headcodes on RTT means that the 'history' of many services
is very difficult to follow automatically as many trains run to (slightly)
differing schedules from day to day and/or week to week.
The sheer complexity of WTT/VAR/STP/vSTP schedules, cancellations,
as required paths, constant retimings, etc, make it one of those situations
where even the most powerful computer algorithm would struggle to make
any meaningful predictions, so ironically, the only trains that it would be
possible to get accurate running predictions for would be the few freights
which run daily anyway!
...likelihood of a train running on a given day of the week based on past performance, how likely it is to be early or late etc.?
That describes exactly what the Freightmaster Online timetables do,
albeit achieved by good old fashioned knowledge and experience,
rather than relying on a computer algorithm...
MARK