“In the car” versus “on the car.” Here the rule is reversed. You are “in a car” if you’re traveling by car. When you’re “on a car” it means you’re on top of it. You also get “in a taxi,” not “on a taxi.”

hasn’t quite become popular yet in British English.

The Online Etymology Dictionary says the first recorded use of “man in the street” to mean the ordinary person dates back to 1831. “Man on the street,” on the hand, the Merriam-Webster Dictionary says, dates back only to 1926.

It is also important to note that “on the streets” (note the plural) means being homeless (as in, “if you don’t pay your rent you will be on the streets”) or working as a prostitute (as in, “The government should devise policies to protect the girls on the streets in our cities”).

“In the street” (note that there is no plural) can also mean “without a job, unemployed,” especially in American English, as in, “After she lost her job at the ministry she was on the street for three years.” The American Heritage Idioms Dictionary says this idiom is attested from the “first half of 1900s.”

3. “On the train” versus “in the train.” When you’re traveling by means of a train, you say you’re “on the train.” That’s the fixed, conventional expression to use in all native varieties of English. Being “in the train” indicates your position in relation to the train (that is, that you’re inside it), not the fact of your traveling by it.

Note that this is different from the idiomatic expression “in the train of,” which is synonymous with “in the wake of,” as in, “many people were rendered homeless in the train of the massive flood.” Also note that “in train” is another fixed phrase that means “wellorganized” or “in progress,” as in, “The report of the recently concluded national conference is in train.”

In short, in transportational contexts “on the train” is the preferred expression.