Midland
Railway Tumbler locking frame
The Midland railway pursued its own ideas right from the very start of signalling, and the Tumbler locking frame (introduced around 1870) was their first design to be built in large numbers.
In a complete departure from other manufacturer's practice, the entire locking frame was built above operating floor level, with the lever catches and interlocking mechanism all encased in a black metal surrounding. This feature had several benefits, apart from cleanliness, in that it allowed maintenance to be carried out in good light and also permitted low or ground level cabins to be built without complication. A side benefit, given that the lever pivoted above signalman's foot level, was that the levers were easy to get a powerful swing on.
This style was perpetuated right through to the British Railways era on the London Midland Region.
Here are two close-up views of tumbler interlocking - the cover plates behind the levers have been removed to expose it.
This example is at Upper Holloway, more details of which can be found in the Photo Gallery. Other examples are illustrated at Bolsover and St Albans South.
Midland Railway Tappet
locking frame
The Midland Railway swung to tappet locking in 1909.
Such frames were very similar in looks to the Tumbler type, although there were minor differences.
The tappet locking, which was much more compact, continued to be placed at the back of the levers.
This example is at Napsbury, more details of which can be found in the Photo Gallery.