The best link that I can give at present is a link to the sources on Github, which is here
. This is rather work in progress (it is difficult to find good information on time interval signalling), but it is an implementation of the means of railway signalling whereby a "danger" signal is given if a train has passed in the last 5 minutes, a "caution" signal is given if a train has passed in the last 10 minutes but not in the last 5 minutes, and a "clear" signal is given otherwise. This dated from the time before there was any means of communicating between different signals whether the train had passed the next signal ahead (i.e. circa 1830s until it began to be replaced by absolute block between about the 1860s and 1880s).
The system worked a little differently at junctions, and it is that part of the code that is currently incomplete and not working, although I do have an idea of how to implement it properly (but it is stuck behind a queue of other incomplete tasks that I am slowly ploughing through).
I am also considering adding an intermediate "time interval with telegraph" system that will allow for time interval working on single track lines, as happened in reality during the transitional phase (1850s-1880s).