> It wasn't so much the car line, it was the cars.

It was a bit of both, more precisely the track and type of truck.

> The older 1000s (originally double-ended but single end at the time I rode them) had a smooth ride but tended to yaw a bit.

The "D-Types" and 1016-1040 were built with with B-3 Trucks. The survivors of each type are still equipped with them.

> Perhaps wheels were worn out (in the 1960s) or a little out of gauge?

No, because wheels were changed regularly whenever the wear exceeded certain standards.

> The newer 1000s (single-ended from the start) were by far the smoothest-riding and fastest.

Same trucks as the D-Types.

The top speed of all three classes of cars, D-Types, 1016-1040, and 1100s, is/was the same because it is a ERPCC/TRC design specification, not a Muni or Saint Louis one.

> Suspect it was the Clark trucks on the 1100s that made the difference. Plus, of course, they were already worn out when Muni got them.

The 1100s had/have the Clark B-2 trucks, which play less well than B-3s on the open track found on the private rights of way and in the tunnels unless equipped with shock absorbers.

Muni did not use shock absorbers on either type of truck, although there were provisions built into both types for their installation. The B-3 has bolster guides, and the Muni used shims to adjust the guide clearances in order to obtain certain limits on bouncing. This means "bounciness" would vary between cars equipped with B-3 trucks. Not only this but the trucks on the same car could have varying degrees of bounciness.

Even when equipped with shock absorbers, the equalizer bars in the B-2 trucks tended to break at the small end when used on open track. The MBTA had a lot of problems with the B-2 equalizer bars because of all its open track.