Comment by iggldiggl

> I vaguely recall it not being compatible with either 25kV electrification or high-speed rail in general, and most modern tracks therefore using axle counters instead.

Track circuits aren't incompatible with that per se, but axle counters are simply easier to install and much more maintenance-friendly – no longer having to worry about

- mixing track circuit currents and traction return currents together

- having to keep the rails sufficiently isolated from the ground and each other to prevent the track circuits from falsely showing occupied

- insulated block joints

- having to use each bit of track once every twenty-four hours to prevent rust from falsely showing a track as clear

- extreme leaf fall and/or sanding potentially causing false clears, too

- length restrictions on the maximum length of a single track circuit, although that's only really a problem on more sparse trafficked lines with long block sections

In return, axle counters have the drawback that they

- don't detect broken rails (although it needs to be said that track circuits very much aren't perfect broken rail detectors, either)

- can be falsely reset (with more or less protections, depending on local operating practices)

- don't detect maintenance vehicles freshly placed upon a track until they enter the next axle counter section

but since most to almost all new installations seem to use axle counters, the trade-offs are apparently worth it to infrastructure operators.