Apple Pay users could be fined if their battery runs out on the train

Apple Pay caused ripples of excitement when it was announced, and just the other day it found its way across the ocean to the UK. The contactless payment method transforms iPhones and Apple Watches into cardless way to pay for low-cost items with little more than a tap.

But if you plan to use Apple Pay to pay for travel by bus, tram, or train in London, it may not all be plain sailing. Using a phone or watch to make a payment is supposed to make life easier, but it could also result in a fine. Transport for London has issued a warning to travelers pointing out that if their battery dies, their journey could prove expensive.

Apple Pay is not unique in that -- obviously -- it can only be used when power is available. An iPhone or Apple Watch is needed, and a dead phone or timepiece is little use to anyone -- including ticket inspectors. Use your iOS device to start a journey and you'd better hope and pray (if that's your thing) that the battery holds out. If you reach your destination with a flat battery, you'll be unable to check out and could end up being charged the maximum possible fare for the route you have traveled. If an inspector is unable to check your device mid-journey, this could result in a fine.

Running out of juice is not the only problem that can arise; Transport for London lists other potential issues. People from overseas or anyone making a payments with a non-UK card may need to factor in an additional transaction fee. If you've linked an American Express card to your Apple Pay account, this cannot be used with Transport for London; make sure you have a backup plan, or you're going nowhere!

It's also worth noting that, unlike other methods of contactless payment, Apple Pay is not instant. Users need to select the payment card they would like to use and authenticate with a fingerprint -- all of which takes valuable seconds. If you receive a call in the middle of making a payment, authorization will be interrupted and you'll have to start again. With a line of people all paying with Apple Pay, a last-minute dash for the train could end in disappointment.

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