Is this a particular turnout and does this happen with any/all of your motive power?

Kinda like this?
http://www.youtube.com/watch?v=hf75Ww_WbX8
That fold-away coupler is sweet.

It looks like it got a little off topic, and I don't think anyone was trying to scare you off with talk of such large radii, any layout, if you've got the space go for as large a set of curves as you can.
Okay; Bachmann's northerns will handle 11.25" radii just fine, I personally have not see one yet that didn't, like any big steam engine though they look 'funny' overhanging these smaller curves but they will handle them.
For double tracking on curves, as pointed out in an earlier post, Bach offers 12.5" to go with their 11.25"  but to help with both of these I strongly recommend using a piece of their 19" radius track sections to transition into and out of all mainline curves, all of your equipment will love you for it be it steam, diesel, or rolling stock.
You'll also want to use multiple power feed points around your layout, because once you've gotten past a simple oval you start to see the power drop off the farther away you get from a single feed point. and for any cross overs between the mains you'll want to be fully insulated electrically.
Hope this helps you out a bit further.

Any rail gaps or misalignments? The plan you show already has the 19" transition curves. Is there a particular spot on the railroad or just going into any of your curves? Which wheels are coming off the track?
One thing to check if it is all the curves is 'side to side' play of the J's drivers, there should be about a 32nd of an inch of side play in each of the 4 driver axles. After that possibly engine tender + binding.

N-Scale Rull-o-thumb; A three axle steam loco of any wheel size, including most articulates, will pretty much handle any curve you throw at it.
But when you throw in an extra axle, wheel diameter comes into play, small wheeled locos like the 2-8-0, 2-8-2 H&L, and 2-8-8-2 should negotiate 9.75" without any hic-ups. But when you start into the larger 'Mainline' power such as 2-8-4's and 4-8-4's, big wheels and a long fixed wheel base almost demand larger than 11" radius.
This not to say that they won't handle such a tight radius but that you will have to expect more derailments from them. The Bachmann 611 spectrum, that I have one of, which runs on smaller than usual drivers handles 9.75" all the time but every once in a while 'hop' and all the drivers are on the ground spinning.
Now one thing all steam engines like is a good 'transition curve' to take you from tangent track into your 9.75" and 11" radius curves, this is best achieved by the use of flex track on sanded cork roadbed but in a pinch the old Atlas trick of including a section of 19" radius curve between the tangent and your tight radius will work just about as well.

Well one good spot to put in a couple of straight pieces is between the crossover turnouts 1 & 2 and the siding turnout 9 at the top of the plan.
The other spot is two places, either just past turnouts 4 & 5 (I'd put the extra track between 3 & 5 on the inner track and to the right of turnout 4.) or between turnout 8 and the curve.
just keep it equal on both sides.

I just got one of these little jewels the week before Xmas. In the JMRI I selected the 4 function mode and I had it accept all of the program choices including speed table (from 5 up to 92 for a max) and dimming of the headlights. Also choice between 'silent drive' and 'precision glide control'.