The Tri-County Metropolitan Transportation District of Oregon (TriMet) this spring will begin the process to replace 26 of its oldest light-rail vehicles.

The “Type 1” vehicles have been operating since light-rail service began in 1986. TriMet will replace the units with vehicles that have updated mechanical and electrical systems, modern safety features and low floors to make boarding quicker, agency officials said in a press release.

The agency plans to start the design phase for the new vehicles sometime next year, with manufacturing to begin in 2020. The new units could enter service as early as 2021.

TriMet aims to retire the Type 1s in 2022. The older units cost 28 percent more to maintain than newer vehicles, and replacement parts for Type 1s are getting harder to find, agency officials said.

The Type 1s surpassed the original equipment manufacturer’s estimated useful life of 1.5 million miles of service per car in 2014. Built by Bombardier, the units are expected to reach 2 million miles per vehicle by the time they’re retired.

The agency is soliciting public comments on the design of the new “Type 6” units. TriMet used public feedback to determine features for its Type 5 units, which were unveiled in 2014.

Built by Siemens, the Type 5 cars entered service with the opening of the Orange Line in 2015.