Other Stadler EMUs (single level cars) don't weigh as much as the KISS (double level cars). There is always a weight limit per axle with railroad cars - golly there's a weight limit per axle in the trucking industry as well. At some point they have to start adding axles.
From the first link,
From October 2018 to Spring 2019, Caltrain will perform work on the four train tunnels in San Francisco to pave the way for Caltrain Electrification and to bring the tracks to a state of good repair. In order to accommodate the new electric trains, crews will create additional clearance and install anchor bolts in the tunnel ceilings to facilitate the installation of the overhead contact system that will power the new electric trains.From Construction Overview – CalMod:
- San Francisco - Brisbane: preconstruction
- South SF: foundations, poles, wires, power substation
- San Bruno: - Burlingame: foundations, poles, wires
- San Mateo: foundations, poles
- Belmont - Menlo Park: foundations
- Palo Alto - San Jose: preconstruction