Calder & Thrushmoor Railway · Box No. 21
A timber-on-brick junction box of 1897, shut in 1968, opened again in 2004, and still worked by the frame it was built around. The locking has never been altered.
Pull a lever and the frame either lets you, or it explains itself. Start with 14, then 13: nothing that moves a train will answer until the gates are across the road and bolted.
The frame says
Frame normal
Every lever is standing in the frame and every signal is at danger.
Calder & Thrushmoor Railway — Hollowgate Junction — locking table
Tappet locking is one relationship read in two directions. A lever cannot be pulled until the levers in its holds column are already standing as shown — and the moment it is pulled, those are exactly the levers it pins there. Both marks the one relationship that refuses to be mutual: a facing point lock has to go in whichever way the points are lying, so 5 holds 4 in either direction and 4 does not hold 5 back. Nothing in this table was typed out by hand; it is printed from the same array of numbers the frame above is working from, which is the only way to be sure the two agree.
| N° | Works | Holds | Held by |
|---|
Nothing in the box tells a driver anything. The levers pull wires, the wires pull arms, and the arm on the post a quarter of a mile away is the only thing anyone on the train ever sees. When we say a signal is off, we mean an eight-foot length of painted timber has moved.
We are twenty-two volunteers and one paid fitter. We do not run trains — we keep one box, and we help other people keep theirs.
Second Saturday of the month, ten until four. Climb the twenty-three steps, stand at the frame, ask the duty signalman anything. Children are welcome and are usually better at the locking than their parents.
Free · donations to the boiler fund
A supervised session working the box under a passed signalman: block bells, train register, the full sequence for a Down train and a branch move. Four people maximum, because there is only one frame.
£45 a head · booked by enquiry
A one-day class in mechanical locking for volunteers, modellers and new signalling staff. Morning on tappets, locking trays and dog charts; afternoon proving a locking table against a real frame.
£120 · six dates a year
We survey, strip, re-bush and re-lock frames for other preserved boxes, and we draw the locking table to go with them. Eleven boxes since 2011, the furthest in Caithness.
Quoted per frame · survey first
The frame keeps the junction safe. The block bell keeps the line safe — one train in one section at a time, agreed beat by beat with Thrushmoor Bank and Calderfoot. Codes are rung, repeated back, and written down before anything is pulled.
| Code | Rung by | Meaning |
|---|---|---|
| 1 | Either box | Call attention. Nothing happens on the block until this is acknowledged. |
| 3–1 | Box in rear | Is the line clear for a stopping passenger train? |
| 3–2 | Box in rear | Is the line clear for an express? |
| 1–4 | Box in rear | Is the line clear for a mineral train? The quarry rings this one four times a day. |
| 2 | Box in rear | Train entering section. The signal has been pulled and the train is on its way. |
| 2–1 | Box in advance | Train out of section, complete with tail lamp. The section is yours again. |
| 6 | Either box | Obstruction danger. Everything stops, and the register records the minute. |
The box is open on the second Saturday of every month and by arrangement for groups. It is a working frame in a listed building, so there are stairs, there is no lift, and the kettle is temperamental.
Open days and sessions