As I recall my Grandfather's words on the subject: He helped construct the cab signal system on the bridge, which was a pure speed signalling set up. The cab signals themselves limited the speeds to 35mph; which was the highest aspect available. Moreover, the Key System articulated cars were under powered for anything faster up the incline portion.

As he put it: "higher aspects were not necessary, nor was more power in the cars; because they (and the SP cars) were so damn heavy that the inherent bridge harmonics could not tolerate higher speeds anyway." Either the cars would have had to be lighter, or the bridge built beefier. Instead, the design engineers settled on 35mph as tops and built everything to that standard.

In my own career as a signal engineer, I discovered another possible rational for the 35 mph limit. It turns out that at commonly achievable on-rail braking rates, and relatively short passenger trains; the sweet spot for track capacity is between 30mph and 35mph. Faster or slower than that, and the possible number of trains per hour would actually go down.

And since at the peak hours, trains were scheduled out of the Transbay Terminal an average of one every 62 seconds; it would have been impossible to keep to the schedule unless speeds were limited to about 35mph.