PENNSYRR.COM by Jerry Britton

ALTO CTC machine right

Union Switch & Signal (US&S) was a primary supplier of control systems to the Pennsylvania Railroad. Their compact, desktop series of CTC machines were labeled the 500 series. What follows is a discussion of what is seen on these machines and how they differed from location to location.


Comments from Lin Bongaardt

The PRR was an extensive user of CTC technology but sometimes in ways that were different from other railroads. On the PRR, Train Dispatchers were not allowed to operate control machines so most were installed in Interlocking Towers, not in Dispatchers' offices.

One of the best examples was "Cola" Tower at Columbia, PA. It controlled the lion's share of the Columbia & Port Deposit Branch and Interlockings on the Atglen & Susquehanna Branch along the Susquehanna River.

One reason that the PRR's CTC usage was not as well known as other railroads' was probably that the use of CTC was not mentioned by name in PRR Employee Timetables. PRR "code" (mild pun here) was that, from the 1941 rulebook, the combination of Rule 261 + Automatic Block System denoted the reverse-running by Block Signal and Interlocking Signal Indications that characterized CTC, as well as Traffic Control System (TCS as used by the NYC) operations. CTC as used by the PRR qualified as a TCS system per ICC and FRA regulations.

It can be noted that the panel light colors vary between electrified and non-electrified territory. Also, it can be observed that panel lights for signal indication are either on the signal toggle or on the track schematic (to date I have not seen both; it's one or the other).

ALTO Tower

Our first subject is the machine that was installed in ALTO tower (Altoona) to remotely control WORKS, ROSE, HOMER, and ANTIS. It features 30 vertical spaces. Above is the right half of the machine.

ALTO CTC machine left soft lg
Left half of the machine at ALTO.

ALTO CTC machine rightRight half of the machine at ALTO.

Across the top of the machine is a diagram of the interlocking(s) under control of the machine. Controlled track is in white and non-controlled track is in outline. The names of each interlocking are across the top.

Occupancy of blocks is depicted with red lamps. Active signals, regardless of aspect, are depicted with green lamps.

Below each interlocking are a number of indicator lamps. It is unclear if they are intended to be the same color or not. "MO" indicates the interlocking is under manual operation from the field (not the tower). There are two lamps for a "Low Air" condition. The higher value indicates that the pressure has dropped below the designated pressure, 45 psi in the above image. The lower value indicates that the pressure has dropped so low that switches must be spiked and wedged before traffic may be allowed to pass over them. "PO" indicates that power is off. "MO" indicates that snow melters are on.

The lower half of the machine features a row of "switch" levers (odd numbers), a row of "signal" (and sometimes "traffic") levers (even numbers), a row of "code" buttons, and a row of specialty buttons.

The switch levers here are a bit different than the norm. They feature three lamps instead of two. The normal position is to the left ("N") and the reverse position ("R") is to the right. Normal is indicated by a green lamp and reverse is indicated by an amber lamp. I am told that the center lamp, which is red, indicates when the switch is locked.

Another variation on this machine is that the signal levers have no lamps. The indication is shown on the diagram as previously mentioned.

Some of the specialty buttons, with the tower's name replacing {tower}, include "{tower} MTR CALL" to call the maintainer for assistance; "{tower} SNOW MELTER" to turn on the snow melters (which lights the aforementioned lamp); "{tower} FLEET} to turn on the fleeting function of the signals, enabling them to act link ABS signals.

More info in this article on The Trackside Photographer.


Excellent article on the installation of CTC at Brady tower, from Railway Signaling, August 1944.

The article indicates that traffic levers have a pair of purple lights. The center lamp above a signal lever is red when all signals controlled by the lever reflects a STOP aspect.

This instance, like ALTO, also has signal lamps on the track schematic and not on the signal levers themselves.


CTC Horseheads

Signal indication is on the signal lever.

KASE Tower



Signal indication is on the schematic.


This installation has the traditional signal lamp indicators on the levers and not on the track diagram.




Rather than recreating the wheel, please see the article US&S CTC Machine Operation in Amtrak's THORN Tower on The Position Light blog.

Note that lamp colors on the panel may differ in electrified territory.


Operator Dick Herr is entering information on the Station Record of Train Movements at the 1938 vintage Union Switch & Signal Co. CTC machine governing train movements from Glen Loch through Coatesville, PA. This type of US&S panel was the technology that first enabled large scale centralization of dispatching or interlocking control. AT&SF, PRR, SAL, SP and UP had large such installations - some panels twice the size of Thorn's 120 lever capacity. Thorn was built in conjunction with the electrification of the PRR in this region. I held second trick here for a number of years, whereas Dick Herr was the first trick operator. Judging from the clock, I was likely working an overtime day as extra operator, owing to some mishap necessitating that a large volume of orders be handed up to passing traffic. The railroad here was still heavy freight territory at that time, with a healthy helping of GG-1s, E-44s and just about anything else under wires. (Rick Erben photo from RailPictures.Net)

ALTO CTC machine left soft lgLeft half of the machine at THORN.

ALTO CTC machine rightRight half of the machine at THORN.



Truxall is on the Conemaugh Division. The photo below is from 1952.

PRR Truxall Conemaugh Div 1952