Mobile data on the metro and in tunnels: what to expect and how to prepare

Why the signal disappears below ground

A phone needs to hear a mast, and a mast on a rooftop cannot reach far into the ground. Ofcom, the UK telecoms regulator, lists basements alongside thick walls and metal frames among the things that impair coverage, and a metro tunnel is an extreme version of a basement. Underground coverage therefore depends entirely on antennas installed inside the system, usually through a deal between the transport operator and the mobile networks.

That is why the experience changes so much between cities and even between lines in the same city. A newer line can be covered end to end while an older one only has signal on the platforms. A travel eSIM connects to the local networks, so where they have equipment underground it works there too, and where they do not, no plan will.

Plan the route before you go down

The trap is the map app. Google says that when you are offline, transit, cycling and walking directions are unavailable in Google Maps, and only driving directions work from a downloaded area. So an offline Google map will still show where you are when you come up, but it will not plan the metro journey from scratch underground. Search the route on the street, while you still have signal, and leave that screen open.

Apple Maps is different: our guide to Apple Maps offline explains that Apple says a downloaded area keeps directions for driving, walking, cycling and transit, where the feature is available. On either phone, it helps to write down or screenshot three things: the line and direction, the station where you change, and the exit you need. Big stations have many exits, and finding the right one is when you will want data most.

Tickets and payment at the barrier

Tapping in with a contactless card or a card stored in your phone's wallet does not need your phone to have data at that moment. On an iPhone, Express Mode lets some transit and payment cards work at the gate without unlocking the phone, in the regions where Apple supports it. Tickets kept in an operator's app are another story: some need to have been opened or downloaded beforehand. Our guide to transport tickets without a connection covers each case.

If you plan to buy a ticket in an app, do it at street level. Doing it on a platform with a weak signal is how people end up waiting for a payment confirmation as the train leaves.

During the ride

• Download a playlist, podcast episode or film before you leave the hotel.
• Send important messages before going down; they will wait and go out later if not.
• Do not start a video call on a covered platform: it will drop in the tunnel.
• On long journeys in poorly covered stretches, Airplane Mode saves battery.

The last point is Apple's own advice for areas with low or no signal: a phone that keeps searching works harder and uses more battery. It is the same reason a day spent hopping between stations can leave your battery lower than a day on the street, as explained in why phones drain faster when travelling. Our guide to downloading music and films works just as well for the metro as for a flight.

Coming back up, and long rail or road tunnels

Out of the tunnel, the phone usually finds the network again within moments. If it does not, Apple's advice for reconnecting is to turn on Airplane Mode for at least 15 seconds and then turn it off, which makes the phone register again from scratch. If the travel line still shows nothing, check that it is the line chosen for data and that data roaming is on for it.

Long tunnels on trains and motorways behave the same way, often for several minutes. On high-speed lines the phone also has to keep changing masts at speed, so a connection on the train is patchy even between tunnels. The train's Wi-Fi, where there is one, often uses the same mobile networks; see Wi-Fi on trains in Europe for what to expect.