Thursday, September 22, 2016

Mechanical Interlocking in Fichtenberg


Fichtenberg is special on the Murrbahn. This is the last station with a fully mechanical interlocking. I've read Stefan Carsten's book about "Mechanische Stellwerke", but so far I have not actually seen one in operation. The actual interlocking machine is located in the single-story annex of the main station building. Placing the machine in an annex instead of a tower was a common arrangement on the Murrbahn.
The "Fahrdienstleiter" manages train traffic through the station in coordination with neighboring stations and the local tower operator.


After switching at Scholz Recycling was completed, I walked over to the station and asked nicely, "Would you mind if I took a photo of the interlocking machine?" The answer came in broad Swabian, "Not at all! Feel free to come in and take a closer look."


Turnouts and signals are thrown with these levers. Each lever locks in either the thrown or normal position. The blue levers on the left are for turnouts, the blue lever in the middle ("Hs2") controls the switching signal on track 2. The red levers on the right control the signals to and from Murrhardt, the next station to the south.
Each lever is connected to the respective turnout or signal with a steel wire loop. Utilizing the wheel at the base of the lever, the movement of the lever actuates the steel wire and thus moves the turnout or signal respectively.

Signal C on track 2 with Hs2 in front. The boxes at the base of the signals contain a wheel mechanism that converts the movement of the steel wire to a movement of the semaphore or the signal disc respectively. Hs2 has not been reset to Stop after the previous train passed through. That's ok, as the main signal takes precedence for regular trains.
In addition, this signal is equipped with a Zs1 ("Ersatzsignal"), an alternate way to allow trains to proceed e.g. when the signal is broken.  


The green box is the "Blockwerk", which is used to set dependencies of blocks between stations and within stations (if a station has more than one tower). The dependencies are locked electro-mechanically, and ensure that no other train can enter a locked path, e.g. the single track between two stations. Block dependencies are released by the passing train when it exits the protected block.


Underneath the Blockwerk are the route levers. Route levers are mechanically connected to the "Verschlusskasten" (meaning roughly "locking box"). Thus, route levers can only be moved if the turnouts are set to the correct position. In combination with the dependencies controlled by the Blockwerk this ensures a route will remain locked for a passing train, and no other route can be set to conflict with the set path.

A closer lock at the route levers


At the north end of Fichtenberg is Tower 2, which controls the turnouts and signals on that side of the station using similar machinery as above. The interlocking in Tower 2 has block and route dependencies to the interlocking shown above, thus the operator in Tower 2 can only set and acknowledge routes requested by the Fahrdienstleiter.


The steel wire loop passes from the levers through rollers and ducting to the respective turnouts and signals. Within the immediate station area or near the tower, the wires are usually run in underground ducts. The square metal box above has rollers than turn the wires 90 degrees towards the tower ("Gruppenablenkung"). If you look closely you can see the ducts that cover the wires leading to Tower 2.
The metal box ("Gruppenablenkung") is barely visible in the previous photo of Tower 2 between the tracks and the tower to the left of the yellow plastic box.


Further out, the wire rollers are often mounted open-air. This kind of roller only carries the wires straight. This is desirable since every deviation from a straight line increases the pull resistance in the steel wire, and thus makes it harder to move the levers.


These pressure rollers allow the wires to follow the curvature of the track. Generally, gentle turns of the wires by 3 to 5 degrees are not problematic with these rollers. When sharper turns are needed in the field, segments of stranded wire can be spliced in.


Signal A is the entry main signal to Fichtenberg station from Murrhardt to the South. The signal can show 3 aspects: Red/Stop/Hp0 as it does in the photo, as well as Green/Proceed/Hp1 and Yellow/Restricted Speed/Hp2. The pre-signal (with the orange disc) indicates the aspect of the next main signal. Signal A is also equipped with a Zs1 armature.