Of Interlockings And Other Machines ...: St.Albans South Signal Box, 2013

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On Sunday, I finally visited the meticulously restored heritage St.Albans South Signal Box – unique because it still stands where it was originally built.

St.Albans Signal Box, 25.8.2013

The four tracks of the Midland Main Line line from London to Sheffield are directly outside the building, and lots of trains are running up and down:

Train arriving on down slow line, St.Albans, 25.8.2013

Levers and train, St.Albans Signal Box, 25.8.2013

In a small garden around the box, a number of demonstration signals are presented, many of them working:

Demonstration signals, St.Albans Signal Box, 25.8.2013

But now we are inside the signal box with its "Midland tumbler frame"—I'll show a few pictures of its curious locking bed later. Here is the whole frame, with Rob at the entrance starting to explain how it works:

Lever frame and Rob, St.Albans Signal Box, 25.8.2013

The track diagram shows the old layout of the station's tracks. All the points have since been removed, and also the storage track shown at the bottom of the diagram (starting at points 27B and 27A). This is, incidentally, the reason that the box could remain at its original place: Because of the removal of this track, the distance to the live tracks is large enough so that a fence and a small walkway could be built, yet the safety distance to the operational tracks is large enough:

Track diagram, St.Albans Signal Box, 25.8.2013

Here is the frame seen from the other side, with a multitude of signal repeaters mounted to the block shelf:

Lever frame, all normal, St.Albans Signal Box, 25.8.2013

Rob set out to explain the necessary actions for a train movement—here is an excerpt from his demonstration (which uses a complex simulator that emulates the actions of the neighbouring signal boxes and the trains):

The following diagram shows the facing point locks as well as the signals that need to be cleared when a train runs towards London on the fast line:

And here is a picture where the corresponding levers are pulled:

Lever frame with up fast route cleared, St.Albans Signal Box, 25.8.2013

The simulator now lets the train "run" through the interlocking:

"Train" running, St.Albans Signal Box, 25.8.2013

In the background of this picture, which shows the pulled levers for the fast line up movement once more—however, the yellow lever of distant signal no.7 has been returned to normal—, one can see five wheels used to adjust the wires of signals far away.

Update Jan 2014: At signalbox.org, there is small thread on—let's say—"non-standard uses" of wire adjusters.

Lever frame and wire adjusting wheels, St.Albans Signal Box, 25.8.2013

Here is a situation on the track panel where the simulator runs three trains at the same time:

Three trains running, St.Albans Signal Box, 25.8.2013

Train on up slow line, St.Albans Signal Box, 25.8.2013

Signal number 10 is the advanced starter for the slow line towards London. Here Rob returns it to its normal position ...

Returning lever for signal 10, St.Albans Signal Box, 25.8.2013

... and goes on to explain other movements:

Rob explaining, St.Albans Signal Box, 25.8.2013

Below this block instrument one can see a "Welwyn crank," which is used for accepting a second train into a line segment that has not been cleared by a first train (usually because the first train never came). Essentially, it is a time-delay device manually operated with a crank that has to be turned many times to force the signalman to think that long about what he is going to do:

Block instruments and a Welwyn crank, St.Albans Signal Box, 25.8.2013

Here is a lengthwise shot of the block shelf above the levers—for an explanation of all the devices, please visit St.Albans and ask any of the knowledgeable members about them:

Block shelf, St.Albans Signal Box, 25.8.2013

Here are two interesting levers, painted with black and white arrows: They were used to place detonators on the lines (in foggy weather, or for any other emergency situation). Incidentally, the corresponding levers for the down lines have the black and white arrows pointing downwards!

Detonator levers, St.Albans Signal Box, 25.8.2013

On the lower floor, a few of the transmission levers have been kept where signal wires or points rodding leading outside was connected:

Transmission levers on lower floor, St.Albans Signal Box, 25.8.2013

And here are a few pictures of the "tumbler mechanics." The logic of a tumbler frame is exactly as the more conventional tappet frames—i.e., it is a type of "cascading locking" (in contrast to route locking as used in Continental Europe). In standard tappet frames, "wedges" are moved by tapered cuts in "tappets," and in turn lock other tappets (see for example the pictures in this posting). In a tumbler frame, the tappets are replaced by axles that turn when the levers are moved; instead of the cuts, there are small levers on these axles; and instead of the wedges, there are slotted rods that are pulled or pushed by the small levers. I stop my explanations here—hopefully, the following pictures and the video show a little bit of that mechanical apparatus:

Mechanics of a tumbler frame, St.Albans Signal Box, 25.8.2013

The electrical wires seen in the next picture are not part of the interlocking itself, but only necessary to connect various parts to the simulator mentioned above:

Mechanics of a tumbler frame, St.Albans Signal Box, 25.8.2013

Before I left the signal box, I took a few pictures of trains running outside through the old levers:

Levers and train, St.Albans Signal Box, 25.8.2013

As the signal box is still near the tracks, one has a great view of the many trains going to and from London. One can see that the fast line curves with a big radius through the station, whereas the slow line has straight platforms so that all doors can be easily watched—however, the radii on both sides of the station are therefore tighter than on the fast lines.

Tracks at St.Albans, 25.8.2013

The new repeater on the fast up line showing green when the corresponding signal is clear can be seen nicely:

Down fast line train, St.Albans, 25.8.2013

Repeaters, St.Albans, 25.8.2013

Outside, I took four pictures of a nicely made demonstration (tappet) frame where members explain how such a more conventional (in the English interlocking world) frame works: