Yo Steve.
1) I've never seen one, but it did get mentioned on my original rules course in 1999. Here in Blighty, TCA stands for Track Circuit Actuator and they're used mostly on On-Track Machines, which due to their low number of axles have a tendancy to go around in stealth mode sometimes in low adhesion season.
Here's what a railway chat site says....
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Originally Posted by ukrailforums
The TCA is effectively a single-turn transformer. There is an incomplete loop of copper tube within the bogie and as stated a signal generator circulates a high-frequency current through this. The other "coil" is the circuit formed by the wheels, axles and rails, which therefore carries a current at the same frequency through all four wheel-rail contact patches on that bogie (or whole vehicle in the case of a Pacer).
This high frequency current reduces the impedance of the contact patch at the lower frequencies (DC up to a few hundred hertz) used by track circuits, and therefore makes it more likely the train will be detected. The TCA is not effective if the rail is fully insulated, for example by heavy leaf contamination, but there is a track-mounted device called at TCAID that can be fitted in the worst areas, and detects the much lower induced current caused by a TCA-fitted train.
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2) Sand. I think we may have discussed this before. IMHO, when you have to put it down, you have to. If we keep disrupting track circuits then maybe the infrastructure operator might actually start to clean the railhead properly. We've recently had a serious collision due to railhead contamination, and I suspect that a certain organisation is going to get a big botty-kick for not cutting back vegetation and not cleaning the railhead during Covid lockdown (whilst trains were still working, even if their lineside crews weren't).
Cheers, John