The route will have "train-like" features to speed up service, such as signal priority, all-door boarding, further stop spacing, and specialized vehicles.

The Twin City Rapid Transit Company operated a streetcar on Selby Avenue and Lake Street starting in 1906.

Route 21 serves the corridor from Uptown Transit Center to the Union Depot in downtown Saint Paul.

[4] Buses on the corridor used to travel on the National Register of Historic Places-listed Selby Avenue Bridge over Ayd Mill Road.

Increased ridership led Metro Transit to leave the detour in place following completion of the replacement bridge.

Unique shelters will have lighting, heating, emergency telephones and security cameras, and station areas will have seating, bike parking, and trash and recycling.

[22] On October 28, 2019, it was publicly announced to the Transportation Committee of the Metropolitan Council that the route would extend past Snelling Avenue into downtown St. Paul.

The recommended alignment does not have the route jog north of I-94 to connect to the METRO Green Line and Midway district of St. Paul.

The rationale behind the decision was that more trips destined for the Lake Street-Marshall segment originated on Selby Avenue and downtown St. Paul than in Midway.