I just recently purchased the Niagara and can run it (carefully) on a layout that has a couple of turns that are less than 18" radius.  Low speed around the curves is important (ha, try telling that to my grandson). My layout is very small (actually folds up into a basement wall, that's why I always drool over those enormous layouts in magazines. The tender is more of a problem than the engine (it's one of those centipede designs and doesn't do well around tight curves but from what you say it may be ok. I'm thinking of buying a different tender to ease this problem.

Paul

I have same DCC system and engines but no problems (now, after finding out that the MRC system will change CV29 settings with engine address changes although none turn off DCC operation some disable DC operations). I assume you checked for AC voltage at the track (per the MRC manual) and if you can make certain the engines run with DC. Call MRC since they do have some people who will go through problems on the phone (and in my case solved the 'problem' quickly).

The MRC website also has a variety of articles to print that can be helpful in trying to understand some of the ins and outs of their DCC system.

Good luck

Problem now solved, thanks to RichG for suggesting that a CV must be out of order.
Yes, both engines did run in both DCC and DC when I first put them on the layout. I'm using an MRC Prodigy Advance2 system and it turns out that when I changed each engines address to a 4 digit road number the MRC system automatically changed CV29 from 6 to 34. After downloading a CV29 table from MRC's website I found that CV29=34 disabled analog operation. Fortunately the table indicated that setting CV29 to 38 would restore analog operation. By the way if I used a two digit address CV29 was automatically set to 2 which also disabled analog. For a while to only thing that worked was to set address at 3 and CV29 at 6 (the original settings), this allowed DCC and DC operation but isn't too convenient for multiple train operation.
So now everything runs as intended,  aren't computers great.

Thanks again

I just received my second DCC On-Board engine (4-8-4 Niagara) to go along with aF& style diesel, also DCC On-Board. I ran both for a while, in both systems. After a time of running in DCC I decided to switch my layout to DC, ran an old DC engine with no problem but now neither of the new DCC On-Board engines will run in DC.

Any idea as to what happened. Both engines run fine in DCC but do nothing in DC