There's crowded and then there's crowded.

Take a Toronto subway train during the morning rush, put it on one of London's busiest routes and they'll get 20-per-cent more people onto it. Put the train in Tokyo and the passengers per square metre will jump to half again as many as Toronto.

These are comparisons that undermine the regular refrain that the Toronto system is bursting at the seams. But they also reflect the highly subjective question of what it means to be crowded.

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"We're used to people touching us on all sides. You just go into a zone where your normal boundaries about personal space get changed," said Lianna Etkind, with the British advocacy group Campaign for Better Transport. "When you're squeezed in on all sides … it's unpleasant. But it's seen as normal. That's a normal part of peak-time commuting."

The amount of space people feel they need varies from country to country and city to city. It evolves with changing human behaviours – a vehicle can hold fewer passengers if they're looking at their phones – and attitudes.

Even within a transit system, passengers will have different ideas of what is too crowded. During the busiest period one recent morning at Bloor-Yonge station, about half of the people waiting on the platform got on each train. But who got on was often the result of individual judgment. Some people would look at a heavily laden train and decide to take their chances on the next one. Others were determined to squeeze their way on, regardless how tight the fit.

This subjectivity around space puts transit agencies in a bit of a bind. Expanding capacity can be hugely expensive, which may seem profligate when the system is able to carry more people in its current form. But transit agencies are also competing for riders and cities want to move commuters out of automobiles, a strategy unlikely to work if would-be passengers believe the system is overloaded.

What is crowded?

For most passengers, crowding comes down to a qualitative analysis. Can I sit down? Open a newspaper? Lift arms to read at all? Can I get to the door when my station arrives? Is the vehicle air-conditioned? Do I have strangers pressed against me? How long does it go on for?

"The phenomenon of crowding has become, not more severe necessarily, but more sustained over time," said Geoff Hobbs, the head of transport planning at Transport for London. "So there are some places which used to have a peak period of only 15 minutes but now they experience a much, much longer peak period."

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In Toronto, subways are less full and the peak is shorter. But it's not because of a philosophical difference between the transit agencies about some absolute concept of crowding.

"What 'crowded' is, is measured by our loading standards," said Chris Upfold, deputy CEO and chief customer service officer of the TTC.

He explained that the new subways on the Yonge line are considered full during peak periods when they're carrying 1,100 people, about half of them standing. The older trains on the Bloor-Danforth line are full carrying 1,000 passengers, half of them standing. Buses and streetcars have their own loading standards. All of these loads – which translate to averages of 4.67 people a square metre at the busiest – are rooted in the funding available, the physical space on the vehicle and efficiency.

"You can easily put more than 53 [passengers] onto a 12-metre [bus]," Mr. Upfold said. "But then, when you start to go above that … you start to have problems with people moving around in the bus. So your dwell times, your [boarding and exiting] times, go up. Getting them on and off becomes more problematic. So it's not just a perception of how comfortable the ride is."

A state of mind

But perception is also important when it comes to how people view the desirability of transit, the TTC and other agencies will readily concede. And a growing body of research shows that a great number of factors influence how people assess their transit experience.

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Perception of time spent waiting depends on the type of transit, for example, as well as the amount of information available, the level of pollution in the area and even the presence of trees. Other research shows that people are willing to stand 20 minutes on transit as long on the service is fast or reliable, but have less patience if the vehicle is tightly packed. And another study shows that the negative perception of crowding is worsened by a lack of fresh air.

Even the way transit issues are reported may have an effect on how the public assesses crowding.

In Melbourne, where the transit agency is removing seats to help trains carry hundreds more than they were designed for, crowding has become a hot topic. And according to Public Transport Users Association spokesman Daniel Bowen, this may have helped spur a backlash to a station proposed near a suburban mall, from critics that included some people seemingly without direct experience of transit.

"If you just saw [crowding stories on the news] and you didn't see the typically less crowded off-peak services, you might assume that the whole network is packed 24 hours a day," he said. "People that don't use the system know it's crowded, but perhaps the perception doesn't quite match the reality, and it may put them off … even trying it."

And seniors, along with disabled people and the infirm, can find a crowded system hostile.

"If you can elbow your way into the doorway then that's good for you," Mr. Bowen said. "Those of us who are able-bodied, we can often squeeze into spaces that others can't. And [it] is important to remember the growth in the system and the growth in capacity does need to cater to everybody, not just those that are completely able to get around without any problems."

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A moving target

Complicating the picture, perception about crowding varies not only from place to place and person to person. It also evolves over time.

According to a 2011 paper published at the site Railway Technical Web Pages, trains for Hong Kong were engineered to be capable of carrying 10 people a square metre, with a lower number set as the transit planning goal. But a 2014 Legislative Council report on the city's transit found that riders' behaviour and tolerances were changing, resulting in trains carrying fewer people than before. Part of this was that more passengers were using electronic devices as they travelled, requiring greater space for each rider. More strikingly, what the report called "passenger riding habits" were also changing.

"Nowadays, they are less willing to board a train that looks crowded even when there is still room available," the report reads, noting wistfully the six passengers a square metre trains traditionally carried. "They prefer waiting for the next train."

Mr. Hobbs said that parts of London's system are at a level of crowding such that waiting for the next train is unlikely to help. The city is now undergoing a major transit expansion, including the huge Crossrail project, but with 100,000 people moving there every year the extra capacity will likely be filled as well.

"In some sense [crowding is] a symptom of success," Mr. Hobbs said. "Success as a city and success of the transit network in being a competitive mode, given that people do have at least a degree of choice about how they get from suburb to city."

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Competing for riders

Transit agencies sometimes talk as if their customers were drawn from two distinct groups: "captive" riders have no choice but to use transit and "choice" riders have to be convinced. It's an analysis that generates substantial backlash from people who see the labelling as pejorative. And research shows that the binary categories also gloss over greater complexities.

A report this year from the New York-based foundation TransitCenter showed that passengers who did not have cars still used a number of other ways to get around. And others have pointed out that people may drive sometimes and take transit at other times. Price, reliability, service frequency, comfort and crowding all play a role in those decisions.

The TTC's Mr. Upfold said the reality of having to convince people to ride is one of the reasons the agency's vehicles are not allowed to get as full during the day. He called those hours "a more competitive market" because people have more choice.

"People will drive more readily, so we have to provide a more attractive product in our off-peak, to get those same riders," he explained. "Making a change on how crowded things are, or how pleasant the experience is, should have a corollary impact on your ridership."