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With temperatures expected to soar above 90 on Friday, TriMet will likely slow MAX trains during the evening commute.

((Bruce Forster Photography/TriMet) (Bruce Forster Photography))

With Portland area temperatures edging past 90 on Monday afternoon, TriMet has warned MAX riders that trains will be running slower than usual.

Oregon's largest transit agency said high temperatures typically cause overhead light-rail power lines to sag and rails to expand, requiring trains to slow in higher speed sections to prevent infrastructure damage.

With 90+ temps, minor MAX delays possible as trains run slower in hi-speed areas to not damage wires/rails. Plan extra time. — TriMet (@trimet) June 8, 2015

Based on experience, MAX commuters should expect delays of 15 minutes or more.

Hard Drive has written in-depth about how heat slows MAX - and why. From the archives:

-- Why does summer heat slow light-rail trains in Portland, but not in Seattle?

-- MAX and heat delays: Every light rail system built for its climate.

-- Science of the commute: Why MAX trains run slower in the heat.

-- Heat slows MAX trains (we wouldn't want 'sun kinks,' right?)

Overheated riders are already getting grumpy on Twitter:

@trimet Minor delays, huh? You call seven back-to-back blue line MAX trains a "minor delay"? — Otto (@ottomotivevlogs) June 8, 2015

-- Joseph Rose

503-221-8029

jrose@oregonian.com

@josephjrose