Railway inspection: watching above, beside and below the track without stopping trains

Railway assets fall into track (rails, sleepers, ballast), civil structures (bridges, viaducts, tunnels, embankments, cuttings and slopes), electrical systems (overhead contact lines, feeders, signalling and telecoms) and stations and buildings. The main drivers of deterioration are repeated train loads, rain and water, and electrical wear.

Japanese railways follow the national technical regulations, and each operator sets its own inspection intervals and methods. For civil structures such as bridges and tunnels, a national notice sets a periodic inspection every two years, carried out under the national maintenance standard (2007), mainly visually, followed by detailed investigation where needed. Track and overhead lines rely on measurement by inspection trains combined with patrols on foot.

The pain points are time and danger. Work on the track is only possible in the night-time maintenance window when no trains run. Each job needs track possession and power isolation procedures and lookouts. Under viaducts are roads and houses, so bringing in an aerial lift also needs road-use permission.

And people. Track and electrical maintenance staff are ageing, and on rural lines a handful of people look after long distances. After a disaster, the decision to resume service must be made quickly but reliably. That is where drones come in.

On a railway the drone’s job is to see without entering the track and to survey the lineside widely. Rather than flying directly over the rails, the norm is to capture from beside the line under a fixed procedure.

Railways pose unusual problems for drones. Overhead lines carry, for example, 1,500 V DC or 20 kV AC, and the strong electromagnetic fields can disturb radio links and compasses. Stations and viaduct undersides block GNSS, and the lineside is full of people, houses and roads.

• Strong electromagnetic fields: near overhead lines, feeders and substations, control and video links can drop. Keep distance and survey the radio environment first. Strong electromagnetic fields
• People below: viaducts stand over roads and pavements. Plan routes that avoid passers-by and decide how to control access. Urban areas & people below
• GNSS-denied: under station roofs, viaducts and near tunnel portals, choose aircraft that localise visually or with LiDAR. GNSS-denied
• Magnetic disturbance: steel girders, rails and heavy currents upset compasses. Magnetic disturbance
• Wind: embankments, viaducts and valley crossings are exposed. Wind & turbulence

Tunnels are a different world — see tunnel inspection. Imaging viaduct and railway-bridge soffits has much in common with bridge inspection.

Railway inspection data is organised by chainage — position along the line. Drone images must be tied not just to latitude and longitude but to which asset at which kilometre and metre, or they cannot feed maintenance planning.

Inspection itself is governed by each operator’s implementing standards under the national technical regulations. Whether drone imagery counts as an inspection record, and which items it can replace, is decided and owned by the operator within its own standards.

On the flying side, Japan’s “specified flight” rules apply. Trains, overhead wires and masts, viaducts and lineside houses count as “property”, so flying within 30 m needs permission, approval or confirmed conditions, as do flights over densely inhabited districts. Flying near the line without the railway operator’s consent is out of the question, and coordination with local authorities and road managers is also needed.

Abroad, the US Federal Railroad Administration sets track safety standards and bridge safety standards (railroad bridges inspected at least once each calendar year, no more than 540 days apart). Some infrastructure managers, such as Britain’s Network Rail, run in-house drone teams with specially trained pilots. Compare flight rules by country in world regulations.

• Precise track measurement: track geometry and internal rail flaws belong to inspection trains and ultrasonic testing. Overhead photos cannot replace them.
• Contact-wire wear values: remaining wire thickness is measured by inspection cars or contact methods. Photos show only appearance.
• Sounding and touch: delamination in viaduct concrete is still found by tapping.
• Over the line in service: flying above an operating line carries too much consequence if the drone falls.
• Operating decisions: speed restrictions and resumption of service are the operator’s responsibility.

On a night of heavy rain, with services suspended, drones lift off one after another from docks along the line. They check high-risk slopes and bridges in a fixed order, and by morning the map shows sections marked “clear” and “check”. Staff go only to the few coloured spots.

On ordinary days drones patrol the corridor bit by bit, their data overlaid with service-train sensors so the track and the lineside sit in one register. Night walks along the line become rarer, and engineers win back time to decide what to fix and when — that, at least, is how the editors see it.