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15 Apr 2024
One of the major issues in building the Internet or any large network is internetworking. If you have two networks built and run by different entities, how do you connect them together? The Internet addressed this problem in two ways. One is by layering, so that consistent upper layers can hide differences in lower layers. (In my office I have devices using wired Ethernet, fiber, wifi, and 5G, all looking the same at the Internet level.) Another is gateways, connecting things togeher and translating the differences, which to some extent is what fiber modems and routers do. But the Internet is hardly the first time these questions have arisen. Back in the 19th century there were a lot of railroads built in a lot of incompatible ways. The most obvious incompatibility was track gauge, the distance between the rails, but there were others including the couplers between the cars and the ways they did (or sometimes did not) ensure that there was only one train at a time on each piece of track. These days most of the world has converged on standard gauge but some countries such as Spain and Russia use wider (broader) gauges, while Japan and some mountain railways and trams use narrower. When a passenger or freight train crosses a border there's a variety of approaches, some of which may seems kind of familiar. The conceptually simplest approach is a gateway. At the border everyone gets off one train and gets on another. The Canfranc station in the Pyrenees at the France-Spain border was famous for this. Another approach is layering. At the border, equipment lifts the car bodies off the bogies of the old gauge and puts them onto bogies of the new gauge. This is better since passengers don't have to get out (often from sleepers in the middle of the night) and goods don't have to be unloaded. This technique was patented in 1876. Here's the Prague-Moscow train changing gauge in Brest, Belarus. Yet another approach is parallel operation, dual or triple gauge, with three or more rails allowing trains of different gauge to run on the same route. In Japan the Shinkansen are standard gauge but older railways are mostly 1067mm so there's a fair amount of dual gauge in and out of cities. This is a very old solution. The Niagara Falls Suspension bridge in 1855 had four rails for three different gauges, although now it's down to two. Here's a video of a dual gauge Shinkansen route. Another approach is switching on the fly. Some trains have variable bogies that can change gauge as the train is moving, which is pretty cool. Here is a Swiss train doing that. And a tutorial in Spanish. The last approach is a flag day. One of the reasons the south lost the US Civil War was that they had a fragmented rail network, which continued to inhibit recovery and development after the war. So over two days, May 31-Jun 1, 1886, southern railroads regauged 11,500 miles of track to the Pennsylvania's gauge (1/2" wider than standard but close enough) and changed the bogies on the rolling stock which let trains run through to and from the north and moved a lot of traffic from ships to trains. Here's a video about it.
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