Don't worry, everyone feels stupid once in a while, like me ().
I thought it was 16.5mm from railhead to railhead. Am I wrong?
Between centers means you are wanting the distance between any two track centers....is that correct? I believe the first poster is very close, although you should consider what the purpose of the track, and the access to it, will be. For example, on a low bench with the two or more tracks right under your belly, you might be able to get away with 2" centres if you don't expect to have to handle the cars or locomtives...that is, get your fingers around 'em. If you have big hands, then consider widening the distance by at least 1/2", maybe more. Also, if in a yard, the #frog turnouts you use will have an impact on the straight tracks intercenter distance. The lower the frog number, you may have to separate them more in order to get a decent curvature coming off diverging routes on those wide turnouts.
If you are talking about hard-to-reach staging, you'll need both elbow room and about 3" between centres as a minimum if you expect to be moving stuff by hand. Otherwise you will very likely derail and knock askew anything near your hands and arms at times.
I model N scale, so this is a bit off the beaten path for me, but it looks like you're pretty close with 2.5". Click here to find out what the NMRA has to say about it.
The NMRA's standard for track centers on tangent and curves is here http://www.nmra.org/standards/s-8.html For tangent there are 2, one for easy handling in yards etc. and one for minimum. For HO these are 2 1/16 and 1 13/16. Commercial turnouts may result in wider centers. Special situations may require wider centers also. IIRC real railroads used 13-16 ft (HO 1.79"-2.21") depending on their situation, era, etc.
Enjoy
Paul
Two and a half inches looks right here, so that's what I'm using. It works OK with my SD40-2 and a couple large passenger cars.
That is probably because there is no fixed number. Real railroads would have various distances between the tracks depending on the situation. If they want high speed trains they have to be further apart than if not. If they want signals between the tracks they have to be a certain distance. If they want MOW workers to be able to stand between the trains they have to be even further appart.
For my modular layout the mains have a two inch separation. I have one industry where there is a center track that is so close to the other two the "dock workers" can put a board plank down and unload the center track THROUGH a car spotted on the outside (building side) track. Probably about 1 9/16" separation.
The classification yard tracks on our clubs layout have 2.25" separation so we can get fingers in between. The main has 2.5" separation so it is obviously separated from the rest of the yard. A passing siding with a passenger station on it is also separated by 2.5"
I personally lay parallel tangent track on 50mm centers, which is close enough to 2" not to matter.
On concentric curves I go out to 60mm (my longest cars are about equivalent to 72' in HO, and I don't have any locomotives with outlandish overhangs.) I get there by using a longer spiral easement into the sharper curve - 20mm offset instead of my more normal 10mm. Minimum mainline curve radius is 610mm (24".)
In some hidden staging, on track which is absolutely bulletproof, my absolute dead minimum center to center spacing is 45mm, the same as the minimum NMRA spacing of 1 13/16".
OTOH, if you are modeling a modern Class I prototype, the UP's standard for new multiple track is 21' center to center, or just a tad under 3" in HO.
Chuck (modeling Central Japan in September, 1964)
Follow the link provide by IRONROOSTER to the NMRA site and their recommended centers. Spacing between parallel tracks is dependent on a number factors such as: straight or curved track, radius of the curved track, and type or length of equipment expected to be run.
2.5" spacing on straight track would be considered rather wide. IIRC, prototypical spacing for straight track would approx 1-7/8" or so. To give you an idea of why 2.5" is wide take a look at the specs of various double track bridges such as the Walthers double track truss bridge. Spacing for the bridge is 2-3/16". Spacing will be wider of tighter radius curves and with longer equipment such as passenger cars, autoracks, big diesel power or larger steam engines.
If you don;t consider these factors your double track mainline may really become a single track main or double track with no passing zones on curves. Then you just need to paint double yellow lines between the tracks on the curves.
Well, out here in the wide open spaces where there's nothing but miles and miles of nothing but miles and miles, the Union Pacific crews that are currently laying a second main line, or previous double-track main lines that were initially laid by the Southern Pacific, are close to 20 feet apart in most areas that I have seen.
The NMRA recommendation of 2 inches center-to-center for HO scale is purely an average based on a collective agreement of their members at some point in time many years ago, and is probably not based on any particular prototype location other than being somewhere in the Eastern half of the country.