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Sherlock avoids public transport where possible, John always knew that.
It wasn't until today that he figured out why.
A rapidly developing case and an unusual lack of cabs lead to the pair of them stuck on a packed tube during rush hour and Sherlock was getting increasingly agitated.
Sherlock took cabs everywhere for a reason, and it wasn't because he was too lazy or too posh, or even that he was quite antisocial.
Sherlock's eyes darted rapidly around the crowded carriage, taking in every detail of every single person, his nostrils flaring slightly as countless odours assaulted his olfactory system, ears registering snippets of dozens of conversations in at least four, no, five different languages set against a tinny bass line from a teenager's mobile.
Too much information.
"I can't think!" Sherlock declared too loudly. "I need to concentrate on the case and I can't think!"
"Sherlock, calm down...." John could see what his friend was trying to do "Stop trying to deduce everyone..."
"I can't just stop..." Sherlock said, as another ten people entered the carriage, and Sherlock's observant mind was at it again.
John got it.
Sherlock spent a small fortune on cabs because he just couldn't cope with so much information in such a small space.
London's public transport system had a tendency to overload his brain.