The Loading Gauge, or: Why the Crane Ate the Train

The harbour crane straddles the quay track so wagons can be loaded underneath it, and it is the model that taught Traluno it needed a loading gauge. It is a portal crane: four legs, a braced frame down each side of the track, a tie across each end to stop the legs splaying, and the slewing jib on top.

The end ties were at train height. Sensible, in the sense that it looked right relative to the crane.

The first goods train through it drove clean into the ties and kept going, wagons passing through solid steel, and the fact that nothing collided was somehow worse than a crash would have been. What was missing was one agreed number for how low anything over a track may hang.

What a loading gauge is

A loading gauge is the largest cross-section a railway vehicle and its load may have, a profile in the air above the rails that nothing running on them may exceed. Its partner is the structure gauge, the space that bridges, tunnels, platform canopies, signal gantries and station roofs must leave clear, and the margin between the two is the clearance (Wikipedia: Loading gauge).

The profiles differ by country, and by a lot. Double-stack container cars, two boxes high, are most common in North America (Wikipedia: Double-stack rail transport), where AAR Plate H, one of the profiles drawn for them, is 6.17 m tall. In Europe many bridges and tunnels are too low for them, and Britain has, in general, the most restrictive loading gauge in the world relative to its track gauge.

The number is not a matter of taste. It is derived from the tallest and widest thing that is allowed to run.

Traluno needed the same discipline for the same reason.

Traluno's loading gauge: one number

The tallest vehicle is the electric locomotive with its pantograph raised: 5.95 units from the rail head to the top of the pantograph. The engine counts in centimetres, so at 1:87 that is a pantograph about 5.2 m above the rail; the scales post has the rest of that arithmetic. Everything else in the fleet, the coaches, the boxcars, the tank and container wagons, sits below it.

The awkward part is not measuring the loco. It is that the number a model needs is not "how tall is the train" but "how low may I hang, measured from my own origin", and those are different questions, because the track and a placed model do not share a zero.

The rail head sits at 1.9 units in the layout's frame. A placed model's frame starts at the ground, the top of the scenery base, at 1.0. So in any model's own coordinates the rail head is 0.9 up, and a structure over the track has to leave that 0.9 plus the loco's 5.95 clear beneath it: 6.85, written down as about 7.0.

That is the whole gauge, as a single number everything spanning track is checked against: nothing may hang below about 7.0 units in its own frame where it crosses a rail.

What it changed

Only two models are allowed to stand over track at all, the harbour crane and the Mountain with Tunnels. A third comes close enough to need the number.

The harbour crane, which the scenery post suggests putting over a quay track laid along the water's edge, got its end ties lifted to 12.2, with knee braces above them. The rest of its bracing, three horizontal ties and a diagonal per bay, runs down the two sides, along the track rather than across it, so the crane still reads as braced without anything crossing the gauge.

The mountain reads the track under its footprint. Over a straight it carves a bore along the line; under a curve, a switch or any other piece it opens the whole cell. The rock over a bore starts 8.5 up, and each stone portal's lintel sits at 8.2, both above the line. It reads the rails and cuts around them, which is the pattern the others should arguably follow.

The station canopy is 6.1 up, lower than the gauge allows over a track, and it gets away with it because it stops at the platform edge. That is the same deal a real platform canopy strikes: it can be as low as it likes over the platform, and has to stop short of the line or go high enough to clear it.

The turntable applies the same idea sideways: nothing static stands inside the 13.2-unit circle its bridge sweeps.

The rule that came out of it

Anything that spans a portal has to clear the loading gauge, and the check belongs in the model definition rather than in the placement code.

That last part is the interesting bit. The instinct is to validate at placement time: refuse to put the crane down if it would foul the track. That sounds safer and is much worse, because it produces a game where you place a perfectly reasonable-looking object and are told no for reasons you cannot see.

Building the clearance into the model instead means a crane can go over any track it is allowed onto. The placement rule only asks whether a model may stand over track at all, and never how high anything hangs. The reason it fits is that it was drawn to fit. The constraint is invisible to the player, which is the correct place for a constraint that exists purely to prevent an absurdity, and the place a failure of it would show first is the cab view, at rail height.

The general lesson

The bug was not "the crane is too low". The bug was that there was no number, so every model author (which is to say, every past version of me) picked a height that looked right for that model in isolation.

Any project with a lot of independently authored pieces has this shape of problem waiting in it. The pieces are individually fine and collectively inconsistent, and the inconsistency only shows where two of them meet.

The fix is never to inspect the pieces more carefully. It is to find the number, write it down where the next author will read it, and go back through what exists. In Traluno it sits in a comment over the crane's end ties, in the file every new building and structure is added to.