What type of railroad three phase delta with only two conductors did I observe?

If the service is actually for the railroad my guess would be it has more to do with the nearby grade crossing arms and lights than for signals for trains. Agree with zbang that they would not put 60hz AC power directly on the track.
Yeah I was just trying to guess why the RR would want a single phase vs a regular split phase?
 
...."split phase" is single phase...

There are limited instances where there is 60Hz power on the track as a grounded conductor, but those are 60Hz traction power systems, like the New Haven-Boston part of the North East Corridor.

BTW and going into the weeds here-
IIRC the original New Haven RR power system was 25Hz traction with 60Hz signal and track circuits. It also derived the power system from wye-connected generation where one leg is grounded as a delta would be and the "neutral point" left floating. All that pre-WW1.
 
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