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.

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. Below is the right half of the machine.

ALTO CTC machine right

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.

Below is the left half of the ALTO machine. Unfortunately the photograph is blurred.

ALTO CTC machine left soft lg