Hi Nathan,
I am a railroader in the usa and have specific knowledge to add.
Reading the documentation is my first order of business to better
understand where we are with tagging. I would like to start with
adding train control like etcs in Germany to the USA rail network. We
of course have other train control systems that may be similar or
different.
Train protection is already tagged in various places in the form of
railway:etcs=yes.
Since there can be multiple train protection systems in place at one
time, this is better than for instance railway:train_protection=xyz
Examples:
https://www.openstreetmap.org/way/503528481
https://www.openstreetmap.org/way/166831834
Also a good place to start is
https://wiki.openstreetmap.org/wiki/OpenRailwayMap/Tagging
Regards,
Maarten