Train service between Heathrow Airport and central London was briefly disrupted on Wednesday morning because of a fire next to a track.

Trains linking Paddington Station with Heathrow’s Terminal 5 may be canceled or face delays of up to 30 minutes because of the fire, National Rail said in an incident report. It did not provide the exact location of the fire or what caused it.

Rail lines had reopened but the disruption was expected to last until 12 p.m. local time, National Rail said in an update.

Last month, a fire at an electrical substation knocked out power at Heathrow and forced the airport, one of the world’s busiest, to shut down for almost a whole day. The closure disrupted air travel for tens of thousands of passengers around the world.

There was no immediate comment from the London Fire Brigade and Heathrow Airport on the disruption reported by National Rail on Wednesday.