Clearance car

Its purpose is to check the clearances around the tracks and ensure that trains conforming to the railroad's standard loading gauge or dynamic envelope will not encounter any obstruction.

[1] Early clearance cars simply consisted of an outline of the system loading gauge attached to a railroad car, which would be towed along the route to ensure the clearances were still sufficient.

These feelers have an advantage in that they bounce back and do not break if they do hit something.

[citation needed] Subsequently, clearance cars using lasers for measurement have come into service.

These are generally HiRail trucks – road vehicles with supplemental rail wheels.

A Washington Metro railcar, originally built for passenger service, later converted to a clearance car. Note the addition of feelers to the car.
A clearance car built to test the ability of impending new locomotives – significantly bigger than their predecessors – to go past platforms and other potential obstructions on the South Australian Railways when the locos arrived in 1926