Train carriages are especially troublesome. They're not only made out of large continuous pieces of signal-blocking metal, but hurtle along at high speeds. Both of these factors make it more difficult for your smartphone to lock on to the signal from a mast or antenna.

Poor phone service on trains is caused by a "triple threat" of physical and technological barriers. First is the "Faraday Cage" effect: modern train carriages are often built with aluminum or steel frames and windows with specialized metallic coatings (to keep the sun out), which act as a shield that blocks electromagnetic signals from reaching your device. Second is the speed and "cell handoff" issue: when traveling at 60 to 200 mph, your phone must constantly disconnect from one cell tower and "hand off" to the next. This rapid switching often fails or causes data packets to drop, resulting in a "dead zone" or a dropped call. Third is the geography of rail routes: trains often run through deep cuttings, tunnels, and remote rural areas where telecommunications infrastructure is sparse or physically blocked by the landscape. To combat this, many train operators are installing "onboard repeaters" or "signal boosters" that use an external antenna on the roof to catch the signal and rebroadcast it inside the carriage. Until these systems are universal, the combination of a moving metal box and remote tracks makes for a notoriously difficult environment for a stable 5G or LTE connection.