Metro bosses have explained why its heaters have been running at full blast in recent weeks.
Nexus, which runs the Tyne and Wear Metro, said it was difficult to control temperature settings due to the age of the technology.
Some commuters have felt uncomfortable after heaters have been running at full blast on carriages, even on milder days.
The existing trains are more than 40 years old and run on "obsolete technology".
The temperature settings cannot be adjusted by flicking a switch or turning down a thermostat.
Instead, engineers must get underneath the carriage and make mechanical adjustments to put the heating system on its summer weather mode.
This means the train stays on winter or summer heat settings for the duration of the day, even if the weather changes.
Nexus confirmed that, as of last week, more than half of its trains have now been put into summer mode.
However, the new fleet of trains that will be rolled out over the next couple of years will have adjustable climate control systems that can be changed with the touch of a button.
Stadler, which is responsible for the maintenance of the Metro, said: “Because the existing Metro fleet is old and parts cannot always be easily replaced, sometimes the heating system does not work automatically and needs to be adjusted manually.
They added: “In the spring, Metro trains are checked one by one to ensure that the winter heat settings are turned off.
"However, not all of the trains that we need for service have had their heating systems altered yet and can feel too warm for some passengers. We are sorry for any inconvenience this causes."
A Nexus spokesperson said: “The new Stadler trains have an advanced climate control system that will allow the ambient temperature to be more easily adjusted at the touch of a button.
“This advanced system will make it easier to regulate the on board temperature throughout the year, with the air conditioning providing a vastly improved customer experience.”
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know...