The French loan phrase en route, pronounced on root, means (1) on or along the way, or (2) on the road. It is sometimes written on route. This form is logical as it conveys roughly the same meaning as en route, but readers who are familiar with the French term might consider it a misspelling. En route is also sometimes written as one word—enroute. This spelling is common enough to have earned its way into some dictionaries, but the two-word form is still more common.
En route has been in English a long time, so it is no longer italicized in normal use (we italicize it here because it’s a phrase presented out of context).
These writers spell en route the French way:
It was en route from the Dutch port of Rotterdam to New York when it reported cracks in its upper deck off the north-west Irish coast. [Irish Times]
Sometimes, when you travel long distances, you may have to change flights en route to your final destination. [Mayo Clinic Guide to Living with a Spinal Cord Injury]
The Mastodons strung together six consecutive victories in December en route to a sparkling 7-2 record. [News Sentinel]
But examples of the alternative forms, on route and enroute, are easily found:
A truck on route to the Mahwah Center for Food Action was filled with 2,900 pounds of food. [NorthJersey.com]
A Pan Am plane on route from London to New York disappeared from radar somewhere above Scotland. [Brand Failures]
After spending the holidays apart, Kristen is most likely enroute to Los Angeles to join Robert at Sunday’s Golden Globes. [Daily Mail]