Setting a train alarm you can actually sleep through

Why a clock alarm doesn't survive a train journey

To set a time alarm you have to know when you will arrive. On rail you don't. A signal failure, a held platform, or waiting for a connection ahead can add fifteen minutes to a journey that was on-time when you boarded — and running early happens too, on a service that skips stops it was scheduled to call at.

A clock alarm that fires early is the worse failure of the two. It wakes you, you look out at open countryside, you go back to sleep, and now nothing is going to wake you at all.

Picking a buffer for rail

This is the setting that decides whether the alarm is useful. The question to ask is not "how far from the station do I want the alarm?" but "how many minutes do I need to be properly awake, get my bag down, and reach the door?"

Then work backwards from the train's speed. A service moving at 100 km/h covers a kilometre in about 36 seconds — so a 1 km buffer on an intercity route buys you well under a minute, most of which you will spend working out where you are. On a suburban service doing 50 km/h the same kilometre gives you a little over a minute.

• Intercity and long-distance. The upper end of the range. Nap Alarm allows up to 5 km, and on a fast service that is a couple of minutes rather than an excessive margin.
• Suburban and regional. Somewhere in the middle usually works, since stations are closer together and speeds lower.
• Check the gap between stations first. The one hard rule: a buffer wider than the distance between your stop and the previous one will fire before you have left the previous station.

If you would rather think in minutes than metres, the ETA mode described on the GPS alarm page lets you set the warning as a time rather than a distance.

When the line goes underground

Plenty of rail routes spend part of the journey in tunnels or deep cuttings where the satellite fix disappears entirely. A location alarm that simply waits for the signal to return can fire well past your station.

Nap Alarm treats two continuous minutes without a fix as a sign you have gone underground and switches to estimating your position from your saved route and its stop timings, handing back to live GPS when the train resurfaces. The mechanics and the limits are set out on the subway alarm page. On a route with long tunnel sections, widen the buffer — an estimate has error in it, and a wider buffer spends that error on being early.

The last-train problem

Sleeping past your stop on a mid-afternoon service is an inconvenience. Doing it on the last train is a different situation: an emptying carriage, a terminus you didn't plan for, and no service back.

This is the case an alarm on its own doesn't cover, because an alarm assumes you wake up and act. If you regularly travel alone late, Trip Buddy escalates to a friend you paired with in advance if the alarm sounds at your destination and nobody stops it.

Before you sleep

• Set the destination station and confirm the map pin is the station itself, not a similarly named place nearby.
• Set the buffer using the speed of your service, not a default.
• Connect headphones if you would rather not wake the carriage — the alarm routes through them when they are connected.
• Lock the phone. Tracking runs in an Android foreground service and keeps going with the screen off.

What this won't fix

• An alarm cannot get you off a train that has already passed your stop. Set it before you sleep, not once you are drowsy.
• If you have never travelled the route with the app, the underground prediction has less timing history to work from and is rougher.
• Rail journeys are long, and tracking a long journey uses battery. Starting a two-hour trip on 8% is asking for trouble.