PPM — the Public Performance Measure — is the rail industry's headline punctuality figure, and it is calculated by Network Rail rather than by us. A train counts as on time if it arrives at its destination within five minutes for a commuter or regional service, or within ten minutes for a long-distance one. A cancelled train, or one that misses scheduled stops, counts as a failure.
The figures come from Network Rail's real-time PPM feed and update about once a minute through the day. Because they accumulate from the start of the service day, the number is volatile first thing in the morning — a handful of early services can swing it several points — and settles as the day goes on. The rolling hour column is the better guide to what is happening right now; the trend arrow compares it with the previous reading.
Operators with no services running report no figure rather than zero, and the feed's own "not available" value is shown blank. Freight operators appear alongside passenger ones.
Operators are ranked by PPM, with the colour showing where each sits against the others rather than against a fixed target. Sectors group operators by the kind of service they run — long-distance, London and South East, regional — which is the fairer comparison, because a two-hundred-mile intercity run and a ten-minute suburban shuttle have very different chances of losing five minutes somewhere.
Freight operators appear in the list too. They are measured against their own booked plan, not a passenger timetable, so a low figure does not mean what it would for a train you could sit on.
Where the feed publishes one, an operator's row opens into its own service groups — roughly its individual routes. This is usually the more useful number, because an operator-wide average is an average of things that have very little to do with each other: a company running both a long-distance main line and a self-contained branch can post a respectable overall figure on a morning when one of them is in serious trouble. The route breakdown is where that shows up.
It is a pass or fail at one point: the destination. A train that runs four minutes late into every intermediate station all day, inconveniencing everyone who used it, arrives within five minutes at the end and counts as a full success. A train one minute over the threshold counts as a complete failure, exactly the same as one cancelled outright.
That is why the industry's own headline number can look disconnected from how a route feels to use, and it is a large part of why the measure has been criticised for years. It is reported here because it is the standard everyone quotes and because it is produced independently of us — not because it is the last word on whether a service is any good.
Our own figures, on the daily archive and operator pages, are measured from the live feeds rather than taken from this one, and they judge delay at the destination rather than at timing points. Because a cancellation is counted here and cannot appear in a delay band there, those pages publish an on-time share both ways — of the arrivals recorded, and with the cancellations counted as failures as PPM does. How that is measured.
Buses are measured separately and by a different method — see bus punctuality and the full explanation of both in how it works.