The real reason is the gap in the tracks where the lift span meets the fixed span. It's about 3 inches wide. Listen the next time you cross on MAX: ka-THUNK ka-THUNK...ka-THUNK ka-THUNK. That's the wheels of the train hopping over the gaping hole in the tracks leading straight down to the river. If they went any faster the train might not land back on the track.

When they built Interstate MAX, there was a line item in the Final Environmental Impact Statement to close the gap and increase the running speed to 25 mph. That was determined to be "infeasible" according to the engineer I talked to at an open house a year or 2 ago.

I feel edified.

OK, but why does the HEAVY RAIL go much faster then?

Yeah, but how come we can't climb to the top and drink 40s anymore?

Heavy Rail doesn't go that much faster, I think it is also limited to about 15-20 mph. But also the construction on the steal bridge that TriMet/Portland/Metro has done to the above sections is much more feeble than the lower heavy rail section. The bridge is maintained by UP and was built originally to act as the main bridge into town for the railroad. The light rail on the upper deck and walkways on the side are all additions since the bridge was created.