With several railroad accidents in recent months, I started to wonder about the tolerance of the distance between the two rails. It can’t be too sloppy or we’d have a lot more trainwrecks. It can’t be too tight or it would be very expensive to lay and maintain the track. My guess would be plus or minus a quarter inch. Anybody know for sure?
Then I got to thinking that even that tolerance wouldn’t be all that easy to maintain for mile after mile. I’m imagining the old days here - manual labor with crews plunking down heavy rails one at a time and John Henry types swinging hammers. So how did they lay parallel track in those days? What kind of machines do they use today?
From my observations, the track is laid out in a rough pattern, but fairly parralel and straight. Hardly final, though.
Then they drive over the track with a machine that forces it in line.
After CSX expanded and replaced rails here, I was surprised how rough the initial layout was. Once the machine rode the rails, the rails were set in place using wood ties and traditional spikes. The ground was tamped in areas to control elevation and keep undulations to a minimum.
I’ve seen Amtrak use concrete ties and non-conventional ‘spikes’ (not even spikes really)
Gauge (the distance between the rails) of existing track can be checked via use of a simple bar with flanges at the ends mounted the correct distance apart, or by progressively more complex means, up to self-propelled railcars equipped with electronic measurement devices.
During modern tracklaying processes, the gauging process usually is automated and highly precise, with laser-guided machinery forcing the rails into the correct position before they are spiked (or clipped) into place. Often the gauge is made slightly larger (by an eighth of an inch or so) on curves.
This page discusses track engineering, but unfortunately does not show a visual example of the gauging process.
The gauge is not as uniform as you would think. The rails tend to be slightly further apart from each other on curves, otherwise the wheels would bind on the curve, and the train would derail. I’m not sure exactly how much tolerance is in this widening on curves, but it’s fairly generous. Half an inch or so. Maybe more. This was even more important in the steam days, when locomotives had long, rigid wheelbases. These days, the wheels are in bogies (trucks), but the extra space on curves is still necessary.
Still, how did they maintain parallel track before they had the machines? It must have been very time consuming to line things up correctly. Either that or they had a lot of derailments.
When newly laying track, the crew would follow a survey line already marked by the surveying crew. Lay down ties (sleepers), drop rail onto ties, Lay gauge bar between rails, wedge rail in or out until in gauge, spike down, move gauge bar a foot or two down the track, repeat. The gauging process is not an excessively time-consuming part of the process when when the spiking is being done by a crew wielding hammers.