"Standard Track center spacing" doesn't exist as a general term. However, prototype railroads do have minimum distance standards between track centers, and this distance may be different depending on the road, and the type of traffic (commuter/passenger/freight) that travels on that particular section of track(s).
Also, to look more "prototypical" many modelers simply assume that less distance between the track centerlines looks more prototypical, but this is not necessarily a fact.
Since I model the Union Pacific's double-tracked mainline between Ogden and Wahsatch from 1947 through 1956, when this corridor saw nearly 75 trains a day (freight, passenger, local traffic) being pulled and shoved by Big Boys, Challengers, TTT's, MacArthurs, Consolidations, E's, F's, PA's, FA's, Geeps, and Turbines...the minimum mainline centerline spacing was (and still is) 20'...which coincidentally in N-scale is exactly 1.5"...the same as Ntrak's minimum track centerline spacing.
Here's a photo of UP mainline trackage on my old Ntrak "Devil's Slide" modules, using Railcraft C70 flextrack and separating the track centerlines at 1.5". Although some critics insist the tracks look too far apart, they
ARE spaced correctly for the U.P.
According to the PDF "Guidelines for Railroad Grade Separation Projects" of the UP/BNSF railroads (dated 1/24/07) the standard "freight mainline grade separation" is 20', and for "freight and commuter" grades, the minimum centerline separation is even larger at 25'...1.875" in N-scale.
Additionally, it took me all of 5 minutes to look this information up on Google using "union pacific track spacing measurements" as the search criterion. I am certain that the same information for many other railroads, both extant and fallen flag, may be just as easy to find.
Here's another point that may be pertinent to the OP's question. Real railroads don't have a problem with running track centers further apart when turnout combinations, crossovers, bridges, diamonds, center sidings, or tunnel bores call for it. The track centers going through the twin curved tunnel bores on my Taggart's module are at 3", smoothly transitioning from a curve on one side of the craggy mountain on the west side of the mountain, curving through it to exit on the east side, then immediately transitioning onto two Warren truss bridges over the Weber River (still at 3" center-to-center) then transitioning down to 1.5" center-to-center at 2" before the edge of the module. My U.P. style center sidings also require the mainline center-to-center distance to expand to accommodate the siding running between them. However, U.P. style double crossovers do not require any extra distance between mainline track centers, so I'm okay at the prototypical 1.5" spacing for them. On my Echo Curve, the two mainline tracks are farther apart than 1.5" because they are at different heights, but they come back together before getting to edge of the module, just as prototype trackage comes back to 20' east of Echo curve, until the mainlines separate again a few miles west of Wahsatch.
I like to see the logical variations in the track center-to-center distance because of various factors on my layout. I think it adds visual and operational interest as well as mimicking what the real mainlines do in Weber and Echo Canyons. So, you might think about doing the same on your layout if your topography and other factors would logically call for it.
Cheerio!
Bob Gilmore
