Most people are familiar with a 'Break Glass' plunger for activating a fire alarm. The electrical plunger is covered with glass to discourage 'nuisance' operation. To press the plunger, you need to first break the 'glass' (these days, it's more likely to be thin, clear, moulded plastic) and the location of the alarm activation is then readily identified by the broken 'glass'.
Something similar, called a 'Sealed Release' is used in Signal Boxes. The most common application is in connection with facing points protected by a Track Circuit.
A Sealed Release in Shackerstone Railway Museum, unmounted (it would normally be fitted to the front of the block shelf above the associated lever). No glass or label is fitted. This example had apparently been installed with the larger side vertical but the examples I saw in the West Midlands always had the smaller side vertical as the sketch below.
Facing points (that is, points which, in the direction of travel, divide one line into two) are dangerous because of the risk of derailing a train if the signalman attempts to move the points when a train is moving over them. Originally, a Locking Bar was provided which usuaally lay below the level of the wheel flanges but was raised by rodding operated from the signal box before the actual points could be moved. If a train was standing or passing over the locking bar when the signalman attempted to raise it, the wheel flanges would interfere with the raising of the Locking Bar and prevent the points from being moved. This was an effective technique but could be cumbersome to implement.
Long before the idea of 'continuous' track circuiting was common, providing fairly short track circuits over facing points could improve safety avoiding the mechanical complexity of a Locking Bar. If the track circuit was clear, the points could be moved - if occupied, the points were locked in position. This was achieved by providing an electric lock on the facing point lock lever in the signal box.
But track circuits could fail and they were carefully designed to always fail in the 'track occupied' state, for safety. So a track circuit failure could stop traffic by preventing facing points protected by track circuit from being moved to the required route. To cater for this occurence, the signalman is provided with a special switch which is operated manually to allow facing points protected by track circuit to be moved during the failure. To prevent casual use of this special switch, the pushbutton is placed behind a glass panel which must be broken before use. This special switch is the 'Sealed Release'. The principal dimensions are given below:-
The Signal and Telegraph Lineman was expected to rectify faulty track circuits urgently and the broken glass would be replaced by pulling the catch inside the Sealed Release, allowing the front half to hinge open. Behind the glass, a paper label was fitted with details of location, attending lineman and date. The paper label was torn out of a small book of numbered labels and counterfoils. An unused counterfoil/label is shown below:-
