A depot is not a running line, and treating it like one is the usual reason a depot enclosure underperforms. Trains in a depot move at walking pace, so the rolling noise that dominates a main line is much lower. What replaces it is a mix of sources that a running line never produces, running on a schedule that a running line never keeps.

Why a depot behaves differently

The sources change

At low speed, wheel and rail rolling noise drops away and the dominant sounds become curve squeal on tight depot track, brake application and release, coupler impact during marshalling, warning signals, and the plant that serves the depot: ventilation, compressed air, washing equipment. Several of these are impulsive or tonal, and both characteristics attract complaint out of proportion to their measured level.

The clock changes

Depots work at night. Stabling, inspection and cleaning happen in the hours when the surrounding residential area is quietest and when most noise limits are at their strictest. A depot that is acceptable at midday can fail at two in the morning with the same equipment.

The geometry changes

This is the decisive difference. On a running line the receiver is beside the track and a wall is enough. Around a depot, the receiver is often above it, because the air rights over a depot are exactly where residential development goes. When the receiver is above the source, a perimeter wall has nothing to block.

Two geometries, and when each applies

Perimeter wall

Appropriate when the depot is at grade and the sensitive receivers are beside it at similar height. It follows the site boundary, is comparatively cheap per square metre, and keeps access roads and drainage intact. Its limitation is that it protects only the receivers whose line of sight it actually crosses.

Full enclosure over the stabling roads

Appropriate when receivers are above, when the depot is surrounded on several sides, or when the limit is tight enough that a wall cannot deliver it. A structural enclosure over the tracks, with absorptive side panels and a roof that may be partly transparent, closes the direct path in every direction along the covered length.

The transparent roof is not decoration. Inside a long enclosure, daylight through the roof is what keeps the working environment usable, and it is also what stops the enclosure from being lit all day at the operator's expense.

The constraints that decide whether an enclosure is buildable

• Ventilation and exhaust. An enclosure that traps diesel fumes or heat is a safety problem, not an acoustic one. Louvre area and mechanical extract have to be part of the acoustic design from the start.

• Fire and egress. Enclosed track needs protected escape routes, and the acoustic panels have to satisfy the fire performance the authority requires.

• Maintenance access. Roof access for inspection, and side access to the track, have to remain possible without dismantling the acoustic lining.

• Overhead line and clearance. The structural envelope is set by the electrification and the vehicle envelope, and the acoustic requirement must fit inside what is left.

• Structure-borne path. An enclosure closes the air path, but vibration travelling through the ground and the building structure is a separate problem. If the development sits directly on the depot, the answer usually involves the trackform, not the barrier.

Where the enclosure stops helping

It is worth being clear about the limit. An enclosure removes the airborne path along the covered length. It does not remove noise from trains outside the covered section, from yard equipment working in the open, or from the structure itself. A depot project that only encloses the stabling roads and leaves the marshalling area open will still have a complaint source, and that is a coverage question, not a panel question.