Not so much...

Dear Ken,

If I can drop into metric for a second:

1 inch = 25.4mm

prototype car width = 10' 8" scale feet wide (AAR "plate" clearance diagrams) 
http://www.icrr.net/plates.htm 

HO scale = 3.5mm/scale foot

Ergo, HO "standard width" car = 37.3mm

Given that 2 cars standing adjacent (hard up against each other, zero clearance) = same as one car width

we're _just_ under 1 1/2 inch (38.1mm)

On sheer maths, 0.8mm (approx 1/30th of an inch) would allow cars to pass,
(like Heavy-machinery movers say, "...an inch is a good as a mile..."),

but the tolerances on typical trackwork,
and varying manufacturer's cars
(any one of which may or may not actually be "prototype width"),
could potentially eat that up in a heartbeat...

(and it only takes a single "sideswipe" to bring the fun to a crashing halt...)

Given what I know of _my_ personal trackwork skills,
(IE no, I would not trust manual track laying by eye to achieve 0.8mm parallel accuracy over any great linear distance),

I would personally aim for 2" (50mm) at a minimum, 
(NB that the prototype transition-era-and-earlier "16' perway" clearance scales out at 56mm,
so even 2"/50.8mm is technically "underscale")

Happy Modelling,
Aim to Improve,
Prof Klyzlr

PS assuming this is a hidden staging situation,
tighter clearances only multiply the pain if _any_ one car derails and has to be manually re-railed/retrieved...
(a rerailer section between every length of flextrack is a reccomended approach)

PPS related images

Yeah, Some older UP yards are

Yeah,

Some older UP yards are 12'-6" centers (they use 13' now) and that's still roughly 1/4" more than you have.

Follow the prototype

The prototype set up their parallel track spacing for a reason. Anything less than 13-foot scale centers is getting very tight. Factor in a little more for any curves.

Eric

 "At this point every 1/4"

1/4 inch everywhere is too close of tolerances to be designing a large layout. Plan on 2" track centers and take the 1/4" out of the adjacent scenery or even the aisle if necessary.....DaveB

In what way is that 1/4" so

In what way is that 1/4" so important?  Is it a physical item that restricts your spacing, such as a stud or duct?  I can't see 1/4" compromising an isle or that small amount of scenery being worth the potential problems.  Can you post up a diagram or picture showing us the area?  Maybe we can help you figure out a better solution.

Do a mock-up

with some spare track, or even use XtrkCAD for a virtual mock-up, but nothing beats the real thing (spare track on some plywood).

Personally I use 2" as a minimum- that's using a 30" or more curve for the inside track, and based upon my use of 80' heavyweight passenger cars.  It's plenty close, but I feel realistically so.  I have a 4-8-2 and an ES44AC as my longest locos if that's any help.  Never a scrape in 6+ years!

Mock up says 1.5" woirks..

Since the tracks in question are straight and only one track will have cars moving at a time.  I was just asking if anyone had any experience with clearances that tight.

As for why the 1/4" matters is the tracks in question are going to be under a mountain ridge, and because this is at the inside end of an aisle with the track making a 180 degree "U Turn" and because of the geometry of the inside corners which MUST be made the same as I want ALL the decks to line up vertically, and to change all five decks at this point would be more trouble than it's worth, the curve for the mainline doesn't leave much room for the scenery for the mountain that is covering the tracks.  If I could space the tracks 1.5" it would make things easier.  I may decide to reduce this interchange down to a one track interchange, but that would mean drastically cutting down the number of cars being interchanged.  I will have to think on this. 

Thanks for the feedback.

Ken L.

Well gee

Or just play it safe and go

Or just play it safe and go with John Armstrongs minimum track centers - usually in yards - which is 2.0 inches.

Run two 180 degree curves

Run two 180 degree curves with your tightest radius using flex track. Run your longest equipment through the area. Distances less than 2 inches happen when building a yard using number 6 Atlas turn outs but I believe it to be 1 and 3/4 inches on center or 1 and 7/8 inches.

Very tight clearances will bring you heartache if your bench work has any seasonal movement that causes the distance to shrink and just might render 1/2 of your yard unusable. On the other hand increasing the distance might only cost a track or two.

Generous clearances will only be a plus later, as will a broader radius used on the layout.

I made mention of the curves as you said your straight tracks run into some turn back curves and by doing this you can be sure there are no impact areas. Keep in mind that if you end up with a car that rocks just a little at the wrong time you could end up snagging and breaking some fine details on some equipment.

You could do wht proto-types do...

Use a gauntlet track.

get commercial track

If you need things REALLY tight, I've seen some commercial track with built in roadbed that's several feet long.  That, at least, could be safer than custom trackwork, unless you're really really good.