5 car Blimp fantrip on American Ave on June 30 1957
When delivered to the Northwestern Pacific (NWP) all the cars had a brake valve with no equalizing piston, which means that train length was limited. When the brake valve has an equalizing piston, train length is limited only by the capacity of the drawbars.
It is, however, standard with all interurban and suburban MU equipment that no more than 10 motor cars may be MUed at any one time because of the current capacity of the control circuits. I thought PE upgraded the brake systems on all of the ex-NWP cars (the one at Perris has been), but perhaps they didn't. I'll have to check around.
All the SP cars (including the ones which didn't go to the PE) were not subject to this limitation.
PE liked the NWP cars because they would go 50 or 55 mph, and they were quite disappointed with the Southern Pacific cars, which were designed to go 43 mph. The ex-SP were subject to a series of modifications to improve their performance. I know of no reason why an NWP car couldn't be MUed with an ex-SP car, but I'm not that familiar with the PE's daily practices.
Caption data from Pete Hinckley; Ken Harrison photo