Prorotype D&RGW C-21 class outside-frame Consolidations (#360 & 361) were reportedly rated at four loaded high-side coal gondolas + a "long" 0500-class caboose on 4% grades.  On the Crested Butte branch, there were curves and frequent precipitation on the rails to make for tough pulling uphill.  Check the Crystal River book for more back-story on the power of these locomotives.  The Bachmann engine has many similarities to the C-21, though it is a scaled-down version.  Motor overload issues aside, my preference would be to double or triple the hill with those 16 cars in the interest of more prototypical operation.  More fun, anyway.

As for the 4-4-0s, mine will start six coaches on tangent track with no signs of overheating.  But any grade really cuts the load capability before slipping occurs.  This is prototypical, too.  Remember when the E&P Eureka ran on the Silverton Branch a while back?  She was maxed out at one Jackson and Sharp coach and one "short" caboose on the steeper sections she attempted.

The photos of the On30 Mogul boiler on the HO 2-10-0 chassis are helpful to visualize the proof of concept.  I agree with the post suggesting substitution of "D" valves for the stock 2-10-0 piston valve cylinder block.  Retaining the Walschearts valve gear would be a blessing for the 1:48 engine mechanics...adjusting inside Stevenson valve gear on an inside-frame 2 foot gauge 2-10-0 would literally be hell on wheels.  Check the Chinese engines.  They may give inspiration for valve gear treatment.

One other suggestion:  Shorten the boiler and/or smoke box so that the center of the stack is directly over the center of the cylinder block (and the imagined blast pipe extending up from the block towards the base of the stack.  This would be consistent with US practice.  Or, Europeanize it with different domes, cab, and external exhaust pipes extended to the centers of the smoke box sides.  Food for thought.

I just received a new On30 outside frame 2-8-0, and it looks great!  However, I cannot find where the two plugs-on-pigtails from the tender are supposed to connect to the engine.  The enclosed "exploded" diagram does not show where the plugs connect.  This has got to be easy, but none of us can find where the plugs go.  Help. please.