Quiet and spacious, with a boxy, Lego toy design, Muni Metro’s new Siemens train cars are a hit among riders who’ve long complained about San Francisco’s overstressed light-rail system.

But passengers have one big gripe: the sideways-facing bench seats are uncomfortable. They’re slippery, they awkwardly pack people together and they cause injuries for people with back problems.

“The seating is a consistent complaint,” Art Torres, a board director for the San Francisco Municipal Transportation Agency, said during a meeting Tuesday afternoon. It drew about a dozen longtime Muni riders who criticized the new seats for undermining an otherwise attractive train design.

“The whole thing is treating passengers like cattle,” said Edward Mason, a Noe Valley resident who rides the J-Church.

Muni’s acting director of transit, Julie Kirschbaum, said she’s heard many such comments on social media and received more than a few cranky letters since the city began rolling out its first batch of 68 new trains. Equipped with long side benches that lack individual partitions, the cars provide more standing space for people to crowd in during rush hour. Yet the configuration has failed to win fans among commuters who want a little more personal space.

So, Kirschbaum presented several alternatives to the board on Tuesday. Muni could ask Siemens to divide the benches into individual seats, she said, and convert one side of the train to forward- or backward-facing seats, like the old Breda trains that the agency is gradually taking out of service.

That redesign would strip out one seat for every quarter of the train, but it might ultimately seat the same number of people, Kirschbaum said. She noted that people are spreading out on the benches and not taking advantage of their full potential.

Several board directors favored a hybrid composition — delineated benches on one side, rows of forward- or backward-facing seats on the other. Kirschbaum would develop a new concept over the next few months and negotiate a change order with Siemens, a German manufacturer with a factory in Sacramento.

She would present the new train seats for a board vote in May, and the first replacement car would arrive in December of 2020.

Muni plans to have 68 new Siemens trains in service by this summer, and 25 to 35 are currently in service on any given day. Ultimately, the agency is replacing 151 Breda trains.

Some riders requested other alterations. They want the seats lowered by 2 inches, so that shorter passengers’ feet will touch the ground. They want more hand straps hanging in the aisle. And they want divots in the seats — rather than the current flat bench surface — to provide “those little curves for your rear end,” Mason said.

The seat issues come as Muni’s leaders pledge to make substantial improvements to the transit agency, long known for delays and sputtering equipment. The new trains are light, roomy and easier to maintain, Kirschbaum said. When a component breaks, engineers can remove it, fix it on a workbench and put it back, rather than taking a vehicle to the shop for days.

For the most part, passengers rave about the smoother ride and the discreet murmur of the Siemens cars — a stark contrast to their clattering predecessors. Yet the seat issues overshadow some of those positive traits, said SFMTA board chair Malcolm Heinicke.

“We’re hearing the current design is unsafe, inhospitable and leads to injury for a portion of our riders,” he said. “I don’t think we can ignore that.”

Robin Krop, a downtown resident who said she rides every Muni Metro line, conducted her own survey of passengers on trains and in stations. She said she talked to about 100 people, half of whom were content with every feature of the Siemens trains, benches included.

“The other half made a face,” she said.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan