Pennsylvania Railroad

by Mike Brotzman

Track Plan

Although at its peak the PRR controlled some 25,000 route miles, its system was in essence a very simple one, consisting of essentially two four-track main lines. One stretched from New York City to Washington DC. The other stretched from Philadelphia Pennsylvania to Chicago Illinois. These two massive main lines met in Philadelphia PA at what came to be known as ZOO interlocking. Named for the Philadelphia Zoological gardens which it literally surrounded, ZOO interlocking was one of the largest and most complex in the country.

Zoo tower
Photograph by Mike Brotzman, 2002

ZOO is a classic (and large) example of the standard PRR interlocking tower design of the late twenties and early thirties. It is built of structural brick with concrete floors and foundation and of course the hallmark PRR rectangular bay window.. There is a peaked slate roof with copper flashing and the wooden framing is painted a lovely sea green. In this modern view we can see that under Amtrak's ownership the queen of PRR towers has fallen on some generally rough times with many of her windows having been boarded up. The name placard has always been rather small (if you can't recognize ZOO tower then you wouldn't be driving a train) and today a small blue Amtrak nameplate has replaced the old, small PRR keystone. Built in 1935, ZOO is an electro-pneumatic interlocking plant equipped by the Union Switch and Signal Co. of Swissvale PA. The PRR bought almost exclusively from US&S, whose headquarters was located right on the PRR main line west of Pittsburgh. The top floor of ZOO tower contains a massive 227 lever US&S model 14 interlocking machine with 135 working levers. This number can be a bit deceiving as each lever on a US&S model 14 typically controlled a PAIR of switches and signals. The first floor is the relay room, containing great bundles of black and green wires. The lower floor contains the air compressor apparatus to operate the pneumatic switch (point) machines.

Zoo tower
Photograph by Mike Brotzman, 2002

In this image you can see the entrance to ZOO. Unlike many British signalboxes, the stairs are completely internal. You can also bet that ZOO had one if not more spacious bathroom facilities inside. In its heyday, ZOO was typically staffed by 7 people including a dispatcher, 2 train directors, 2 assistant train directors and two levermen. The bay window contained a large desk for the directors to work at, with the interlocking machine and model board to the rear of them, facing the front of the tower. Everything was of course track circuit controlled, which made things a tad easier, bit given the fact that ZOO had to interface with no fewer than 5 other towers and one other railroad made the top floor a din with routes and trains constantly being shouted out.

In grand PRR fashion, ZOO was a multilevel "flying junction"\ but even so it made all other flying junctions look downright pathetic. The history of ZOO began in 1876 when the PRR constructed the Connecting Railway to bypass congested street running in the heart of Philadelphia. Instead of accessing the city directly from the north, the Connecting Railway made a sharp turn to the west, crossed the Schuylkill River on a grand stone arch and iron truss viaduct there is then made a sharp turn south where it met the PRR's initial east-west trademark Main Line at what was known as Mantua Junction. Mantua was a typical Y junction contorted by 3 manual signal boxes and with an engine house in the center of the Y. By 1888, the straight leg of the Y (the Main Line) had grown thick with yard tracks and by 1910 the Schuylkill River bridge was enlarged to 4 tracks and all stone arch. Also in 1910 the first two duckunder tunnels were added to allow trains to access the Connecting Railway from without fouling the Main Line in either direction. In 1935 the interlocking reached its final form in conjunction with electrification and the downtown 30th St and Suburban Station projects and the present tower was constructed. The new station projects added new dynamics to the interlocking. The through tracks at 30th St were at river level, while the Suburban level (crossing the through tracks at 90° and crossing back over the Schuylkill River to terminate downtown) was about 40 feet above them. The Connecting Railway bridge at the north bound mouth of ZOO was about 60 feet above the Schuylkill River, so the River Line (as it was called) had to make a sweeping clockwise turn to rise in grade. In the crescent shaped pocket it made between itself and the River was the Philadelphia Zoo, which made an obvious choice for interlocking's name.

Signal at Zoo
Photograph by Mike Brotzman, 2002

While this image does not show ZOO tower itself, it does help to illustrate the complexity and vastness of the interlocking. ZOO tower was located in the center of the Y junction, with a large electric substation right behind it. Upon ZOO converged a four-track main from the North, a four-track main from the west and four two-track lines from the south. Not only this, but to the north of the north leg of the Y was built the Mantua Freight Yard, with 25 tracks and a capacity of 366 cars. Between the west leg of the Y and the Y center were the 37th Street yards with 10 tracks holding 162 cars. Finally, flying under both freight and passenger mains, a connection with the Reading RR entered into the fray from the north. The routing combinations were enough to drive any signalman insane. From the North you had the four-track main to New York handling both freight. Long distance passenger and two commuter routes (NORTH PHILADELPHIA tower). To the West you has a four-track main line handling both freight, long distance passenger and a VERY busy commuter route (OVERBROOK tower). The the south you had three lines. The River Line that served the new 30th Street station for long distance trains headed south (PENN tower). You had the High Line, which was an elevated freight bypass for freights headed south (ARSENAL tower) and you had the line to Broad Street station (pre 1957) and Suburban station for primarily commuter and some regional operations that terminated in Philly (BROAD tower), not to mention the Powlerton MU car yard leads. In the above image we are in a SEPTA commuter train about to pass under lever 78L signal hung on a unique bracket mast that probably pre-dates the 1915 electrification of the Main Line and the position light signals that came with it. This train had already passed through one of ZOO's sub-interlockings at the lever 142L signal at the junction of the Pittsburgh Subway and automatic intermediate signals 28 before coming upon this sub-interlocking where trains bound for the lower level of 30th Street station take the diverging route and trains bound for the upper lever keep going straight. There will be one more sub-interlocking where the Paoli commuter traffic joins the Trenton commuter traffic at a flying junction. Most trains take about three to seven minutes to traverse the interlocking. You can also see a classic PRR amber position light signals, which will be discussed separately.

General view of Zoo tower and signals
Photograph by Mike Brotzman, 2002

Both of the two four-track mains to the north and west had to send trains to any of the three city terminal lines and vice-versa. ZOO also had to handle moves directly from the North/South main line to the East/West main line, moves into and out of the 37th Street, Mantua and Bellmont yards AND interchange moves from the Reading Railroad connection. To handle this spaghetti of conflicting moves, ZOO interlocking had no fewer than 4 flyover/tunnels. The 36th Street tunnel that took passenger moves from BROAD and sent them to/from the Northbound mainline. The River Line duckunder that allowed Southbound trains to duck under freight and commuter moves in front of ZOO tower. The Reading duckunder that allowed Reading RR trains direct access to the High Line and finally, the New York-Pittsburgh Subway that allowed mainly NY-Chicago trains to bypass the Philly terminal and make a hard right turn to the West. In the above image you can see the different route ducking and weaving around each other.

Today ZOO is a shadow of its former self. After the consolidation of Northeast rail operations under Conrail, most of the freight was moved off of the former PRR electrified main line between New York and Washington. This main line was sold to Amtrak and was turned into the high-speed rail corridor known as the NEC. With the long slow freight trains gone, much of what made ZOO such a complex interlocking was largely eliminated. The freight yards were torn out and are now filled with rotting ties and weeds. The High Line was single tracked and connected directly to the old Reading RR interchange line with all connections to Amtrak owned trackage and ZOO tower control being severed around 1995. Two of ZOO's remote satellite interlocking that served to "preprocess" trains heading into the flyovers were spun off as their own interlocking, MANTUA and GIRARD, and had control given over to the remote CTEC dispatchers. Today, Amtrak is steadily working to replace the venerable US&S A-5 pneumatic switch machines with US&S electric M3 machines. However, for the foreseeable future, ZOO will remain an actively manned interlocking tower. Amtrak lacks the money to even replace the old Model 14 machine with something solid state and since ZOO's functions would require a separate operator anyhow, it would be pointless to spend money to move "ZOO" into the CTEC center located one mile away in 30th Street station.

About the photographs

Comments about this article should be addressed to Mike Brotzman