It's not even the longest in Canada. And do branches count?? The Expo line is currently 36 km long - but that includes the the 7 km Production Way-University branch. When the new extension in about 2030 opens on the Surrey branch is 16 km, it will make the longest service 45 km. Which is a bit shorter than the 47 km for the Deux-Montagnes (to Brossard) branch of the REM. For the Expo line if you include branches it's 52 km - but the REM with branches is 67 km.
It will also (barely) shorter than TTC's line 1, which should open not long (LOL!) after the Expo extension, and will clock in at 46 km.
And depends what a "metro" and a "line" is. The RER-like Toronto GO service will be quite extensive. The Oakvile to Oshawa Lakeshore service, which will have the core (and most) very-frequent service, is about 84-km long. Even Oshawa to Union is over 50 km.
Do branches count? The Expo line is currently 36 km long - but that includes the the 7 km Production Way-University branch. The new extension on the Surrey branch is 16 km - making the longest service 45 km.
Meanwhile, in the USA, the Los Angeles Blue (A) LRT line is about 78 km. The Silver metro line in Washington DC is 66-km long. The longest one-seat ride on the complex New York subway is 52 km on the A train - through 3 different boroughs - and that doesn't include the branch. In the San Fransisco area there's 5 different rapid transit lines that are over 55-km long ... some more metro-like than others.
Honourable mention for Chicago, which has two quite long lines - but slightly shorter, both being about 42 km.