First up was this Trackmobile 3050. It may have been what was available on short notice.

At 30,000 pounds, the Trackmobile 3050 is about 12% the weight of the U23B it replaced. It is powered by a four cylinder, turbo-charged, diesel engine.

The Trackmobile can be operated remotely by the same worker who aligns track switches. NASA uses much larger Trackmobiles to move rockets onto launch pads.

Each rocky hopper can weigh about 8 times as much as the Trackmobile 3050. VM often moves three hoppers at a time. The hoppers don't need to be lifted, of course, but rather rolled. A human can push a loaded hopper if the rolling resistance is low and footing good.

A motive unit's ability to push or pull is called its tractive effort, something largely determined by its weight, horsepower, and wheel traction. The Trackmobile's drive wheels are made of steel, while its rubber tires permit the unit to also operate off rails.

Here, emptied hoppers were being pushed into a line for pickup by CSX.

The worker climbs aboard upon completion of handling that day's delivery.

The 3050 Trackmobile apparently proved too light when rain slicked the rails. Two weeks after the delivery above, a worker was testing the unit at a switch, where it could be prone to wheel slip.

The smaller 3050 seemed to run and hide when a bigger Trackmobile challenged the following month.

The Titan model packs almost twice the weight of the 3050, and more than twice the horsepower.

Even so, by April, both trackmobiles had left the ring...

... because a blood-red Nordco Shuttlewagon had arrived with still more horsepower and about twice the weight of the Titan.

The Shuttlewagon was still on duty ten months later, so it would appear VM has found a unit that can go the distance.

This sly machine can round-up four hoppers...

... without breaking a sweat, though it has been known to drip sand onto the tracks.

The Shuttlewagon's traction comes from four rubber wheels.

Like the Trackmobile, it can be operated remotely.

Trying hard now, and pushing empty hoppers...

... all the way back to Philly, well, at least to MD 32. The blue U23B coaches from the corner.

After sunset, it, too, must rest up to fight another day.

Link: PRR's Rubber Tired Switchers