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Braking distance is the space interval required by the heaviest train running at its maximum speed to come to a stand, in which connection allowance has to be made for gradient variations and the adverse effect of a possible greasy rail. In the past a rough and ready computation was to take the braking distance at half a mile, which might be varied slightly according to local circumstances. Whilst this distance sufficed for speeds up to approximately 60 m.p.h., drivers of trains running at higher speeds have to depend on sighting distance, which then becomes of paramount importance, and the necessity for the best possible view of a signal will be realised when it is remembered that a driver running at 90 m.p.h. has only approximately eleven seconds to sight a signal a quarter of a mile ahead, and as his ability to pull up in answer to a "caution" signal varies roughly as the square of the speed at which he is travelling, it will be appreciated that keen sight and alertness on the part of our express train drivers play no small part in the safety of operation.

As an aid in the direction indicated, it has been the practice in the past, on important express routes, to provide very high signals, as shown in the illustration, with top arms often fifty feet above rail level, and lower co-acting arms as a help to sighting when visibility is poor. In clear weather this arrangement leaves little to be desired but, as visibility decreases, the difficulty of sighting the signal increases, until a point is reached where speed has definitely to be reduced.

On the main line out of Euston this difficulty is accentuated by the fact that where four tracks exist there is generally only six feet space between adjacent lines, which does not allow sufficient clearance to permit of a signal being fixed between the tracks adjacent to the road to which it applies, and therefore it has to be fixed outside all lines, as illustrated, the left-hand signal applying to the down fast line, and the right-hand one to the down slow line, i.e., the third line from the left.

An innovation in signalling practice was brought into use on the main line in October last. This consists of mechanically operated colour light distant signals capable of displaying yellow and green lights.

Most readers of the MAGAZINE will be familiar with colour light signals, which although only introduced for main line working in this country about twelve years ago, are multiplying rapidly, but hitherto mainly have been used collectively in complete installations, as at Manchester and St. Enoch, where they are controlled electrically.

Distant signals at Boxmoor

With the introduction of the grid, power supplies are now becoming available at many points alongside the line, and the opportunity is being taken, as renewals become necessary, to replace the old semaphore distant signals with the latest type colour light signal, which with a consumption of 24 watts, provides an effective beam candle power of approximately 35,000. The light from these signals can be readily seen on straight track at 1,000 yards on a bright sunny day, with the tremendous advantage over the semaphore type of signal that, the worse the background and atmospheric conditions, short of a thick fog, the better the signals show up, and even in a thick fog they are clearly discernible on passing, as they create a coloured aureola on the surrounding haze.

In order to obtain the maximum benefit from the use of these signals, it is essential to fix them so that they are as nearly as possible at a driver's eye level, and as close to him as clearance requirements permit. The very best position is as shown on the left in the above illustration, but as such a signal may not be fixed in so narrow a space as six feet, the distance between the adjacent lines, the only alternatives are to fix it either on a gantry spanning all four lines or, as shown in the lower illustration, as a dwarf signal, this latter alternative only being a practicable one where the approach to it is almost dead straight, as on a curve a train on an adjoining line would hide it from view.

The photographs of the signals illustrated were taken prior to them being brought into use; they are the up slow distant on the left, and the up fast distant as a dwarf between the down slow and up fast tracks, so that each signal is immediately to the left of the driver, whether running on the slow or fast lines.

These signals are the up distant signals for Boxmoor, and replace the two tall signals, which can be seen on the left of the tracks, 387 yards ahead, the distance of the new colour light signals from their respective home signals being 1,400 yards on the fast line, and 1,350 yards on the slow line which, on a falling gradient of 1 in 335, is sufficient for a speed of 70 m.p.h. when visibility is nil, that is, in thick fog; whilst adequate sighting distance ordinarily will permit of any speed within the capacity of the locomotive.

Colour light dwarf signals at Boxmoor

It will be noticed that each signal has three lenses, although only required to display the two aspects, yellow for "caution" and green for "clear". In signal engineering, involving, as it does, safety of operation, little or nothing is left to chance, and, therefore, as a safeguard against a sudden failure of the electric lamp in either of the two main aspects, what is known as an auxiliary yellow light is provided, which automatically comes into action immediately either of the other two lamps burns out, and at the same time the message that this has happened is conveyed to the signalman, a buzzer ringing in his signal-box, drawing his attention to the fact that a light has failed and, by pressing the button at the bottom of the indicator case, he can tell which signal it is, as the small lamp immediately under the labels will fail to light up, and the buzzer can then be silenced by moving the switch below from A to B. The lineman is then sent for, and on a new lamp bulb being inserted in the defective signal and the circuit restored, the buzzer will again sound until the switch is put back to its normal position, i.e., from B to A, so ensuring its readiness for similar duty.

Up to the time of writing ten examples of such colour light distant signals have been brought into use; at Cheddington, Tring, Northchurch, Berkhamsted and Boxmoor, in connection with the 1936 signal renewal programme, and further similar renewals are contemplated, so that there is every reason to anticipate that in time there will be a chain of such "beacons" stretching right along the main line, aiding our drivers to comply with our Chief Operating Manager's laudable ambition of being always "On Time", quite irrespective of weather conditions.