46U - here is a little lo-down on the MP and huts.
I spent 31 years w/ AT&T, mostly in private line data/voice. When you drive down roads, highways, etc., you will see the small silver huts positioned along side the tracks. On top you will normally see a 1/4 wave VHF antenna. This is a remote station. The MP is, as you said, the Mile Post marker on the rail. We would get an order from NS, CSX or others to install a voice line at, say, MP 148 in East Bugaloo (that is the location that is listed in the FCC records). We would pass that info to the local Telco to install that portion. Most of the time we had to give them directions like "take highway 48 east out of East Bugaloo, go to Henderson Road and turn right. Cross the tracks, turn right on the dirt service road and travel 3.6 miles to the hut". They would get there and run the cable pairs to a block (usually an RJ21X or RJ45) or even just leave the 4 wires bare. We would test out the line and when all test were complete, we would connect that line to the NS, CSX or whoever else's circuit it was so they could go out and connect a two-way radio to the lines. Take for example CSX. Their southern region dispatch is in Jacksonville. They have hundreds of these private lines around the south. When a train needed to contact dispatch, all they had to do is key up and the the dispatcher would be able to hear them all the way down in Jax via the private line. If the dispatcher wanted to talk to a specific train, they would look where on the tracks they were, select the closest relay station and talk away. Most all of the signal lights are also operated that way except instead of a voice line, they would use a data line.
Make sense?
