At major junctions and busy stations or yards, it has long been the practice to provide a signal tower configured in such a way that two trains cannot be cleared onto conflicting routes, i.e. brought onto the same piece of track at the same time. The railroad term of art for such installations is "interlocking," and I illustrated a simple use of the principles with science-project switches.
We visited a more involved interlocking tower (our British colleagues call them signal boxes) at St. Erth, preventing conflicts between trains for and from St. Ives with the Penzance service for Plymouth, London Paddington, and the all-day cross-country train to John O'Groats (or is it Glasgow these days?) Much of the hardware appears to be early twentieth century, and it's still up to the task of spacing the Inter City 125 diesel trains that are still protecting schedules on God's Wonderful Railway.
On last week's excursion to Pennsylvania points, I stopped at Harris Tower, the interlocking tower that controlled the west throat of the Harrisburg station of The Pennsylvania Railroad. It's now a museum, and it was open on a sunny Saturday afternoon, and there were a lot of freight trains running. That was after placing a reservation at a swap meet for a proper model of a boxcar with a famous number, PENNSYLVANIA 65000.
The museum is soon to be closed for the season. Bear in mind that the railway preservation demographic is aging, and it might be wise to inquire before making a visit. The nearby bridges allow for photography of the railroad from public grounds near the Commonwealth Capitol and state office buildings.
What makes the tower distinctive as a railway preservation effort is that the machinery still works the way the designer intended.
The operator is lining up a route. In the background is the model board, showing the west throat of Harrisburg Station as it was before Penn Central, Amtrak, and Conrail. The platform tracks are at upper left, the freight avoiding line enters at lower left (although The Pennsylvania Railroad could get passenger trains around Harrisburg without interfering with the passenger traffic) and the short tracks at center right held steam locomotives awaiting their assignment to take over The Broadway Limited or The Duquesne or the Middle Division mail train from the electric locomotives.
The track diagram is also present on the computer screen at lower left. The computer is running the now-discontinued Train Dispatcher simulation. Outputs of the program go to relays in the tower's original relay case, which provide inputs to the interlocking machine the operator is using. It's now possible to control even these complicated tracks at a distance, with a computer, and you accordingly don't see many manned interlocking towers. Further, as we shall see, the actually existing track layout is trimmed, substantially, since 1942, the era being simulated. It makes for a more instructive visit, as the computer, running a fast clock, is generating outputs that announce the arrival of trains, either by loudspeaker or by a bell ringing, activate the block occupancy circuits of the approaching trains, and, once the route is lined up, simulate the passage of the train through the controlled territory (old-school rails will know this as the plant.)
The gauge on top of the cabinet monitors the air pressure in the interlocking system. On The Pennsylvania Railroad, a switch lever activates an air valve, and compressed air moves the switch points. The same system is still at work in Chicago's Tower A-2, over which all the empty equipment moves for Metra's Milwaukee, North Central, Union Pacific, and Heritage Corridor pass enroute to or from downtown stations, as do service trains on the Milwaukee, North Central, Union Pacific, and Amtrak. And the cars headed to the chocolate factory!
In an interlocking plant, the operator cannot clear signals to put trains on conflicting routes. It's still up to the engineer to respect the signal indications, particularly the ones specifying STOP AND STAY. These days, we do that work with solid-state logic (AND, OR, NOR, NOT) but the logic can also be implemented mechanically.
A section of the interlocking machinery is covered by thick Plexiglas, which allows the visitor to observe what's going on, although on a sunny day there's a lot of reflected light, including some of the model board. When the operator rotates one of the levers, each of which is covered by one of the thick Plexiglas bars, it activates one or more of the sliding bars that run from left to right in the picture. That rotation plus sliding either locks or releases other levers in the machinery. It's more complicated than the double-pole double-throw switch that I used in the earlier interlocking primer, and laying out the frame takes a lot of thought, as you don't want to install, let alone cut notches and add projections, on more of those sliding bars than you have to. (Time permitting, you'll see how it's done in O Scale, one of these years.)
As an additional cue to the operator, the rotation of a lever illuminates an indicator light on levers that have been released by its movement, or extinguishes the indicator lights on levers that are locked. With the plant in the current configuration, for instance, levers 11 and 13 can be moved to the right (they are currently set to the left, toward a westbound train), lever 12 can be rotated either to the right or to the left (that's a switch whose position does not adversely affect any train otherwise cleared into the plant), lever 18 can be moved only to the left, lever 22 can be moved only to the left, although on some other configurations, it can be moved to the right, and levers 14, 15, and 16 cannot be moved. The "Rusty Rail" marker on lever 19, controlling a signal, warns the operator that the machinery might not pick up the presence of a train on the track protected by that signal.
What, then, about the trains? I got there during a busy period for Norfolk Southern running freight trains.
Here, two freight trains meet on what remains of the avoiding lines. The support poles for the electrification went long ago, with Conrail ending electric freight operation in 1981 and with higher vertical clearances being desirable for autoracks and double stack containers, two betterments unknown to W. W. Atterbury or James Symes. What's less evident is that somewhere near here, the old Conrail main line goes from The Pennsylvania Railroad to taking a ride on the Reading. Amtrak own the Main Line from Harrisburg to Philadelphia, and the Cold Spring Shops Free Rein to 110 is being implemented, with electric operation of the Keystone Service trains.
The Pennsylvania Railroad never invested in extended-vision cabin cars, nor have I somehow found a time machine. Rather, that's a War Department escort car, there were two depressed-center flat cars with DODX reporting marks on the head pin, and these heavy haul cars ahead of the buggy.
And the one passenger train on the Middle Division and Mountain Division leaves Amtrak for Norfolk Southern, about five minutes late.
There's some sentiment in Pennsylvania for an additional train or two to Pittsburgh, and is it really the case that there's no daily sleeping car service direct from Philadelphia to Cleveland and Chicago?
On the eve of Amtrak, Penn Central were still operating The Duquesne and The Juniata for and from Pittsburgh, The Broadway Limited, The Manhattan Limited, and The Pennsylvania Limited for and from Chicago, plus The Admiral from Chicago only, and The Spirit of St. Louis combined with The Cincinnati Limited for and from ... work it out! Most of these trains offered food service (perhaps closed overnight between Pittsburgh and Fort Wayne) with setout sleeper for and from Pittsburgh and Indianapolis still available, as well as sleeping cars for and from Chicago on The Broadway Limited and The Pennsylvania Limited. In those days, the trains swapped diesel locomotives for electric locomotives at Harrisburg.