The SFMTA is warning commuters that trains could feel especially crowded Thursday as it takes all the new fleet of Metro trains offline for emergency repairs. Also, it's still raining, so...

The official word is that they "expect minimal impact on capacity" but they're nonetheless putting the word out that there will be an increase in one-car trains Thursday due to "equipment availability." The Chronicle reports that this is due to a newly discovered problem with the new fleet of Siemens light-rail trains, of which 68 have rolled out since last year.

The problem is with the shear pins — which are one of two ways that the train cars are held together in two-car trains. And much like Muni discovered an issue in the spring in which the new train cars' couplers were failing, they now say that the shear pins appear to only last three months before becoming prone to failure.

The SFMTA says that this did not and does not pose a risk to riders, because the trains are programmed to automatically stop if two train cars become detached. But they pulled all the new trains out of service Thursday to replace the pins, and SFMTA spokesperson Erica Kato tells the Chronicle that the agency is instructing Siemens to "re-design and re-engineer" the pins so they don't have to be replaced so frequently.

The next batch of Siemens trains is not due to go into service until 2021, and the last of the batch of 68 trains just got delivered last week. In total, Muni has purchased 219 trains at a cost of $1.2 billion, and the full fleet won't be in service for about six or seven years. In the meantime, we will still be riding on 148 Breda cars, many of which are only 20 or 22 years old, but which are already showing their age.

As many pundits and experts have discussed over the years, SF's Muni Metro system has a number of unique challenges, not the least of which is the city's topography. (As Muni engineer Emmanuel Enriquez told the Chronicle this week, running up hills or making hairpin turns like these trains have to make in traffic could "rip a train apart" if it wasn't designed correctly.) The trains have to operate on a hybrid above-ground and underground system in which five lines merge into one tunnel, which often causes backups and scheduling problems.

Another issue that may get addressed in the coming years is the fact that the Muni Metro tunnels were not designed with any pull-off pockets or ways to quickly clear disabled trains — which is the most frequent issue causing rush-hour delays.