Updated 1/10/2016:
While thinking about the track geometry for the western mainline rebuild, I came across another “side project” that needs my attention. And as long as I “have the patient on the table”, I figured I should address it.
As trains travel west across the bridge (northernmost track) to Ohio, they encounter an MTH PRR signal bridge with a 7-light position signal at its western end. Beyond the signal is a straight section of track that enters a tunnel. The straight track section has an insulated rail that triggers a DZ-1008A relay which in turn activates the signal. When the track ahead is occupied, the signal displays the STOP aspect. Otherwise, it shows the CLEAR aspect.
Within the tunnel is Switch No. 3 (yeah, I know that wasn't a good idea). This is where the Hidden Passing Siding joins the Mainline. Again, the only direction of travel possible is counter-clockwise.
Currently, the signal protects the approaching train only against an occupied Mainline track. It does not protect against Switch No. 3 being set against the Mainline. That is something that I want to correct, as a derailment or collision inside the tunnel would spoil everyone’s day.
Switch No. 3 is a Curtis HiRail O54 right-hand switch operated by a Z-Stuff DZ-2500a switch machine. The switch machine is activated by either TMCC (using the DZ-2001 Data Wire Driver) or the DZ-2502P pushbutton mounted in the fascia. I cannot use the non-derailing feature of the switch machine as it is possible for a train from the Hidden Passing Siding to be occupying the switch when another train approaches via the Mainline. Throwing a switch while a train occupies the points would be bad, very bad. Adding a provision to cut power to the Mainline straight when the switch is set against it seems like overkill and doesn’t modify the signal aspect. I prefer the engineer to be in control of the approaching train and be watching the signal for instruction.
Here's a rough drawing of the area.
Anyway, I am looking for a way to address this. Current thinking is to use a second DZ-1008a relay activated by the DZ-2500a switch machine and just have both relays activate the MTH signal. I'm going to play around with a possible wiring diagram and post it later this week.
I've got this problem going in a thread on the Electrical sub-forum.
George