I've been on ebay and have seen auctions for HO train track clearance cars...what are these and do I need one?
Are they used to make sure my tunnels are not too short/low?
A track clearance car is used to check the clearance around and above the track. You can do the same with an NMRA Standards Gauge.
Here's a Wikipedia entry about a track clearance car.
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Red Horse wrote: Are they used to make sure my tunnels are not too short/low?
Tunnels, bridges, rock cuts, buildings, signals, water towers, mail cranes, etc. Any fixed object along the railroad.
Modern railroads do it with a hirail equipped with laser range finders. It becomes critical when moving hi-wide loads or large cars (auto racks, double stacks, long/big engines).
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