In this Part 7 of my 2009 PRR Main Line Survey I will cover the segment between ALTO and CP-UN. Now while I did devote an entire
special post to ALTO tower, my Main Line survey is not simply going to skip over that part like it was some sort of black hole so we begin again at the east home signal of ALTO tower at the end of the platform of the Altoona Amtrak Station. Here you can see the applicable track diagram.

Again if anyone is interested in the official interlocking diagrams you
can download them here.
http://www.multimodalways.org/docs/rail ... %20Charts/
You'll want the 1998 Pittsburgh Division one.
ALTO marks the transition from a two track railroad running under Rule 261 on both tracks, to a three track line with single direction Rule 251 on the outer tracks and Rule 261 on the center track. ALTO was arranged, or possibly re-arranged back in the Penn Central era, to allow easy access for Amtrak trains in both directions to access the station platform(s) located on the north side of the tracks. The 31 switch allows westbound trains to continue either on the 251 track 3 or the 261 track 2. Eastbound trains are given a straight shot to the platform off of track 2. Passenger trains heading east on track 1 must first cross to track #2 at SLOPE. Anyway here my train departs the Altoona station and takes the #31 switch set reverse to track #3.

Here we see the reverse direction 20R dwarf signal for movements on track 1. This signal is controlled from the main machine in ALTO tower. To the right we see the helper pocket track and its eastbound 32R dwarf signal. The pocket exists between that signal and the PC era signal gantry.
Ironically one of the most iconic structures at ALTO appart from the tower itself wasn't even original to the interlocking. The 5-track beam type signal gantry was installed during the Penn Central era in conjunction with a PennDoT bridge and highway project in the area. The gantry supports three high signal masts governing westbound movements on tracks 1 and 2 as well as the helper pocket which are worked from levers 2R, 6R and 8R on the main machine.
ALTO tower itself posing by the westbound lattice signal gantry which probably dates from when the tower was built in the early 20th century. Sometime in 2008 the tower was given a fresh coat of gray paint, but this investment was not a signal of any sort of longevity as you all now know the tower is scheduled to close sometime in the next year.
Here we are looking at the 6 track eastbound gantry at ALTO, which only supports two high signals for tracks 1 and 0. The slow speed diverging aspects on both are for movements onto the yard leads which are in fact signaled to CP-WORKS as a controlled siding.
Here we are looking at the 1sw and my extension the entire ALTO facing point ladder which runs from here to the yard leads at the other end of the interlocking. As ALTO is basically a yard-terminal tower most of its movements involve trains either passing straight through or heading into/out of the yard, hence the ladders that head to that side of the track. As expected ANTUS interlocking can be considered the reverse of ALTO.
Here we see the eastbound home signals for ALTO on tracks #1 and #2. As expected the reverse direction track 1 signal is a dwarf and the bi-directional track 2 signal is a mast. This zoom view provides an interesting prospective on the layout of the entire interlocking.