The Midland "Tumbler" Interlocking Frame

The Midland "tumbler" interlocking frame was invented, manufactured and exclusively used by the Midland Railway Company of England. It was introduced around 1870 and was superseded in 1909 by a tappet frame of similar external appearance. A significant number of tumbler frames survived into the later 20th century, and are preserved in museums and similar locations. They were introduced very shortly after the invention and patenting of catch-handle locking by Easterbrook in 1867, licensing the patents of Saxby and Farmer, and the Midland was the only main-line company to do so. From their long and exclusive use by the Midland, one may infer that they were completely suitable and robust. The term "tumbler" has been used for various different things in relation to interlocking, as in the Webb tumbler interlocking. These are quite distinct from the Midland tumbler, and have nothing in common with it.

The absence of technical information on the tumbler frame is remarkable, and one reason why I am publishing this article. It is not mentioned in the famous 1874 article by Rapier, nor in the Railway Engineering Magazine's extensive review of railway signalling in 1882. It is also absent from the contemporary works on Railway Signalling, such as those by Barry and Dawnay, as well as in more modern texts, even as historical notices. Google and Wikipedia searches also bring nothing up, except for the mentions in John Hinson's Signalbox and the preserved St. Albans South Signalbox sites. Neither of these give any technical information, though The Signalbox gives two good photos of the interior. I have never come across a drawing of the tumbler frame, which could tell so much about it. I also do not know the name of the designer of the frame, who should be credited, and the date of any patents involved. All the frames appear to have been manufactured at Derby, together will all other Midland signalling equipment.

Thanks to Mark Higginson of the Derby Museums, I have been allowed to examine the small tumbler frame in the Silk Mill Museum. This explained a great deal to me, but I did not gather as much information as I should have; I did not measure any of the parts, nor carefully note the operation of the catch dog and lever stud with relation to the tumbler. This frame had only a small part of the locking in place--most had been removed and was not present, so it was not possible to gain much information about it. Nevertheless, it was a valuable experience. The photographs in this article were taken at the Derby Silk Mill Museum of the frame on display there.

In the remainder of this article, I shall present my understanding of the mechanical operation of the tumbler frame and compare it with the more familar and well-known mechanisms. The terminology will be my own, since I have no access to the actual terms employed, and the explanations will also be my own, deduced from my observations, which may be in error. My hope is that I shall eventually be able to compose a more authoritative version with the help of those with better information, and I invite all such contributions.

Two levers in a small tumbler frame are shown at the right, as seen from the front of the frame, where the signalman would stand, with the covers removed. The green lever, which we shall call lever 1, operates a distant signal, while the red lever, which we shall call lever 2, operates the home signal. Before yellow was adopted for Caution (in the 1920's) the colour for a distant lever was green, not yellow. Lever 1 has a tumbler on its right, while lever 2 has tumblers on both sides. The studs on lever 1 are clearly seen. The one on the right will engage with the "wing" to the left of the left-hand tumbler when the lever is pulled, rotating it to the right. The left-hand stud in lever 2 is inside a recess in the right-hand tumbler, preventing it from rotating. When the catch dog of lever 1 attempts to rotate its tumbler, the connection between the two tumblers, seen at the top, will prevent the rotation, and the catch will not be released. This means that the distant cannot be pulled off when the home is normal. When lever 2 is reversed, this restraint is removed, so lever 1 can be freed when the catch dog rotates the two tumblers together, and the distant can be pulled off. Now, however, the tumblers cannot rotate because of the action of the stud on lever 1, so the latch of lever 2 cannot be released, so it cannot be returned to normal, which would result in a clear distant in rear of a stop. This very simple locking is expressed as "1 locks 2 reversed".

The tumbler frame is constructed inside a cast-iron housing that is divided into two chambers separated by a plate containing the lever quadrants. The shaft that is the fulcrum for the levers is supported in the lower chamber, while the locking is in the upper chamber. The entire frame is above the operating floor. The cast-iron work is remarkably intricate and required a very high order of skill. The levers are one-piece forged steel of rectangular section. The catch handles are in the back, and the catch rod is square and the same width as the lever, so they appear as part of the lever in photographs. The catch handles are spring-loaded, and the spring is hidden inside. There is space for a tumbler on each side of a lever, but one is installed only when required. The tumblers are cylindrical cast-iron pieces rotating about a horizontal axis, with arms extending on both sides for attachment of the locking bars. The arms have through holes at one or two levels through which pins are inserted between pairs of arms. Tumblers are connected, and made to rotate simultaneously, by steel locking bars extending from right to left, jogged to clear other tumblers. The pins fit in holes or slots in the locking bars.

Another view of the same two levers is shown at the left. The studs and latch dogs of lever 1 are clearly shown, as is the locking bar. Note that the left-hand tumbler can rotate independently, but then the locking bar will prevent rotation of the right-hand tumbler. The locking bar at the top comes from a point lever, and will prevent the home signal from being cleared when inappropriate, though I have not followed this out completely.

The cover for the frame consists of cast-iron plates fitting between each lever that are easily removed so that the locking can be seen and maintained. The tumblers are clearly of different forms depending on their uses. It would be good to know the different forms available.

The catch rod ends in a T-shaped catch dog. There are square notches in the quadrant on each side of the lever at the normal and reverse positions, into which the vertical stroke of the T fits, preventing the lever from moving. The horizontal stroke of the T contacts the tumblers on each side, and can rotate them when the catch handle is pulled. If the tumblers are prevented from rotating, then the catch dog cannot leave its notch, and the lever cannot be moved. If the tumbler can rotate, then its motion when the catch handle is depressed is communicated to the tumblers beside levers that are to be locked, preventing sufficient motion of those catch handles to release the levers. This is the very important preliminary locking function of the mechanism. This motion is insufficient to release the catch dogs of those levers that will be freed when the movement is complete. This ensures that the function will be properly completed before previously locked levers are freed. These requirements are necessary for a safe interlocking frame, as first recognized by Saxby and Farmer around 1867, and which their gridiron and rocker frame of 1874 exemplified.

There are short cylindrical studs on each side of a lever at the level of the tumblers. These engage with "wings" on the tumblers that rotate them as the lever is moved. This motion completes the locking of those levers that are to be locked, and releases the levers that are to be freed, close to the end of the lever stroke. Unlike in a Saxby and Farmer rocker frame, releasing the catch handle does not move the locking the final half. The precise action depends on the actual form of the cam surfaces, which I have not been able to examine. The studs also can fit into recesses that prevent a tumbler from rotating (this cannot be used if the tumbler also prevents a catch dog from rising).

The reason early locking and late unlocking are important is the danger of pulling a second lever when the first lever is only partially operated. This is unlikely when there is only a single leverman, except in cases of horseplay or maliciousness, but can certainly happen when more than one man operates the frame. Many early interlocking frames were subject to this hazard, such as the Lane interlocking on the Great Western or the Toucey and Buchanan interlocking in America.

A Midland tappet frame, as introduced in 1909, is shown at the left. Although of similar external appearance, it is very different inside. The catch rod dogs now drive a typical Saxby rocker, pivoted below and driving the tappets directly. Part of the tappet bed can be seen at the upper right, with the transverse locking bars crossing the notched tappets in the usual way. Clearly, when the catch handle of a lever standing at normal is depressed, the rear of the rocker rises, moving the tappet halfway and locking conflicting levers. The locking does not move as the lever is pulled, but when the catch handle is released when the lever is reversed, the front of the rocker moves down, and the tappet completes its full motion, releasing levers that no longer conflict. One reason for introducing a tappet locking is probably to allow conditional locking (e.g., "1 locks 2 normal when 4 is reversed") that is easy with tappets, but probably not possible with tumblers.

The history of interlocking, and the tappet frame, are discussed in Interlocking. More references are given there.

Photographs of tumbler locking are found in John Hinson's Signalbox

Composed by J. B. Calvert
Created 1 December 2006
Last revised 2 December 2006