It's the Paris commuter line everyone loves to hate and now it's taking even more flak after three passengers decided to start recording the sticky conditions of the trains during rush hour as the mercury continues to rise in the French capital.
“We do this morning and evening always using the same measuring device at the same stations, at the same time,” said one of the passengers behind the project who travels between Orsay-Ville in the Essonne department of Ile-de-France and Châtelet every day.
And the results, which have been posted on one of the passenger's Twitter accounts will put most people off taking the RER B, at least until things calm down a little.
On Wednesday night temperatures reached 37.1C on one train and the thermometer, which also records humidity, showed that in these conditions the temperature actually felt like 43.8C.
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“Over time it gets tiring,” said one of the passengers. “In the morning we are already soaked, already sweaty. I make sure I leave work early in the evening to avoid the rush hour. Luckily, I have an understanding employer.”
The results may come as a surprise to anyone aware that RER has a “refrigerated ventilation” system which, although not exactly air-conditioning, should help cool the air in the carriages.
According to the RER B official blog, 91.7 percent of the trains on the line come with this ventilation system.
However one of the passengers responsible for recording the temperatures on the trains isn't convinced.
“These are sweeping statements,” he said. “Since the introduction of summer schedules, there are fewer trains, fewer services and therefore more people on the trains. They have also been using the MI84 trains more often… and they have not been renovated [with the ventilation system].”
RER B chiefs did acknowledge that travelling conditions were “particularly difficult” in the heatwave and admitted the ventilation systems on the trains “were not optimal in the hot weather”.
Staff were on hand however to hand out water at certain stations on the line.
And it's not just those travelling on RER B who are battling with the heat, with passengers on other RER lines, Transilien trains and the Metro struggling to cope in the current temperatures.
However the situation is set to slowly improve, with transport authority Île-de-France Mobilites planning for 700 new or renovated trains with ventilation systems to hit the network by 2021.
But for commuters they can't come quickly enough.