>1. An acquaintance has told me that his family and neighbors along the tracks used to get electric power from an interurban line that was 110 VAC at 25 cps. But I've understood that interurbans typically ran on 600 VDC. Also he says there were no conversion stations anywhere nearby. So does this make sense, and if so, how did this conversion take place?

Possible because many streetcar systems used 25 cycle AC for distribution to their substations (the T still was doing this in the 1980s), where it was converted to 600v DC with M-G sets for the streetcars. It might have been that the railway's franchise included stipulations about supplying power to residential customers, and that power would have been at the railway's distribution frequency.

And, BTW, 60 cycle AC won't work in a traction motor because it will fry the commutator (induction), which is the reason for using 25 cycles (or 15).

>2. Since interurbans and streetecars ran on DC, and DC can't be transmitted very far (contrary to AC)

The rule about transmission of electric power is the higher the voltage, the lower he current, which means less in the way of heating losses. AC's advantage for transmission is very easy to covert between voltages, just us a transformer. DC is a different matter. While at high voltages it transmits more efficiently than than AC because there is no skin effect, it's problem is voltage change: the conversion equipment is very costly, although solid state devices have brought these down a bit.

>, how was the DC voltage maintained at considerable distances from the power stations?

Feeder cables from a substation to outlying points (in parallel with the trolley wire) and distribution lines (3 ph AC) supplying substations with conversion equipment (about every 5 miles with 600v). A [power] "station" contains a generator, a "substation" contains only conversion and/or distribution equipment.

The Napa Valley Route started began life as a 25 cycle AC road at 750 volts. This was changed very quickly to 3,300v.

1,200v DC was GE's answer to Westinghouse's AC traction system. Eventually this becamce 2,400v (BA&P) and then 3,000-3,300v (MILW).

From what I've learned about Little Joes and their control systems, that 3,400 volt minimum "limitation" sounds like an urban legend because IRM runs their 1500v South Shore one on 600v just fine. I suspect the 3,400v thing had more to do with compensating for line losses and the current capacity of the overhead system etc. when they were drawing a lot of power.