The images I see of Midwest electric interurbans are of one or two car consists. This would be for carriers like Lake Shore Electric. Were train sizes limited by the amount of juice the power plants could deliver to enable making scheduled speeds?

Yes.

No, they were limited by the ability to sell tickets. If they could have sold the tickets, they would have found a way to provide the power.

Interurbans were by definition designed to provide less-than-trainload service to smaller markets.

A lot of interurbans hauled short freight trains with electric motors (such as Iowa Traction today). The power was there to support longer trains.

When Philadelphia, Pennsylvania organizations chartered 3 car trains on the former Philadelphia & Western Rwy., they had to pay for employees to babysit the substations that routinely powered 2 car trains. Large electric railroads had special instructions ordering following trains stopped close to each other to wait 30 seconds after the train in front had started moving before taking power.

Under both Red Arrow (PST) and early SEPTA, P&W had one manual substation; the rest were automatic and unattended. The substation operator worked weekday peak AM & PM only; otherwise they relied on the other substations.

Fantrips were on weekends; if an organisation chartered 3 cars or a Libertyliner, PST/SEPTA had to call the substation operator and the club paid for him. NOTE: most P&W stations couldn't platform 3 cars.

Neither PRR nor RDG had that 30 second rule; block signals would separate stopped trains. Besides, a road that could run ore trains with 3 E44's pulling and 2 E33's pushing up to Paoli didn't worry about enough amps for MU cars.

Phil Mulligan

So, if I'm reading this right, the interurban sized the power supply for the expected demand and enough juice to maintain the schedule. Added demand could only be carried with intervention, and that only so far.

The FDDM&S (an Iowa interurban) had enough generating capacity at its Fraser plant to sell electricity to industries and communities that were along the route. To give themselves some electrical "wiggle room," normal operating practice in general was to run the steeplecab-powered freight trains of any tonnage at night... when the "retail" use of electrical power was lowest.

Something to keep in mind is that the term interurban captures everything from lightly built lines where freight was an afterthought to lines capable of hauling significant volumes of traffic. 

For an overview of the industry, I suggest the book The Electric Interurban Railways in America by George Hilton and John Due. A preview is available via Google Books here: google.com/books/edition/The_Electric_Interurban_Railways_in_Amer/J2zH-zcuU-MC?hl=en&gbpv=0

The best history of a specific interurban is in the book The Illinois Terminal Railroad: The Road of Personalized Services by Dale Jenkins. Here is the Amazon link: https://www.amazon.com/Illinois-Termina ... 1932804005

Jenkins provides a thorough overview of electric freight operations and the motivation for dieselization. The Illinois Traction Society https://www.illinoistractionsociety.org/ has published articles about the role of substations and portable substations in the operation of the railroad.

It depends.

Perhaps unsurprisingly, an interurban with its own separate 'power stations' needed to size generating capacity not only to the anticipated peak car load (assumed as the maximum acceleration of all cars in a section -- see the Gibbs and Hill scaling factors for the PRR electrifications -- but also for what could be hellacious line loss in the DC with increasing distance from the power taps or substations. Unsurprisingly many of these powerhouses ran on comparatively high-voltage AC from power alternators, and sized the substations relative to anticipated demand, weather issues, etc.

Perhaps the greatest savior of electric interurbans past their initial great dieoff by the early 1920s was that rascal Sam Insull, who invented the idea of selling electricity for many more consumer purposes than just light and a little refrigeration. This involved a massive construction of power stations, dams and reservoirs, and distribution architecture, which in turn meant substantial unutilized capacity for the enormous initial stranded cost. That in turn made it very logical for utilities to buy and operate electric transport of all kinds -- relatively immune from any temptation to diesel bustitution.

Of course when the New York bankers took Insull down, and got the government to divorce utilities from ownership of transportation entities...