Locomotive in middle of train

Is there a restriction on trains with a locomotive(s) in the middle of a train (cars on either end)? I remember about a year ago when a MARC train was disabled on the Penn Line and had to be pushed by another train, but apparently since a locomotive was in the middle of the train the speed was restricted to around 10 or 20mph. I've also been on a southbound train that was disabled on the Brunswick Line outside of Silver Spring, and the train behind us pushed us, but again we were only going 10-20mph. But, just last week I saw a northbound VRE train with non-revenue 3 Sounder cars (for return to Seattle) on the rear of the locomotive and the revenue gallery cars in front of the loco, but there didn't seem to be any restrictions on the train. Do MARC and VRE (or Amtrak and CSX) have different policies concerning this?

Another VRE example:
http://youtube.com/watch?v=Twm2DEdSNeM

If I was to guess, what you saw was a push pull set-up that going northbound was pushing, and was dragging the sounder cars behind the loco. It was being controlled by the engineer in the lead gallery car. The other examples you gave were all "rescues", where there was never an intention that the loco in the center would be pushing and not under the control of the engineer in the lead car (or dead loco), but the engineer in the loco in the center would be in control but with no visibility to the front.

If VRE has a mechanic problem with their locomotive and they unable to repair it onsite, the next train that arrives will couple with it and push it in to Union Station. Usually, northbound trains are pushed by the locomotive, so their would the one locomotive in the middle from the disabled train.

However, in the YouTube video I'm not sure what they are doing, perhaps bringing in some cars to the mid-yard for work. I've seen it before on Train #304. They rarely do this.

Just to expand on the mechanics of running multiple sets together, the MBTA (Boston) regularly operates deadhead moves (and, IIRC, some revenue moves) with what they call "double drafts," two complete trainsets. With locomotives at one end, and cab cars at the other, the push-pull trainsets can essentially operate as a pair of locomotives MUed together -- since, in a nutshell, that's exactly what push-pull is doing ("MUing" a locomotive to a cab car).

Not sure if its standard MBTA practice to have both units online on one of these moves or not.

When Amtrak ran the "Mail 12" they would often have the Springfield Shuttle of that train, 412, run with the locomotive on in the middle. The Locomotive was on the south end coming into New Haven and the Control Car on the North End. Mail 12 would cut mail cars and 412 would just couple up to them to the engine and go north.