One engine won't pull much up hill. Two engines will pull about fifty cars up a grade and near a hundred on level track. I think it's how easy your rolling stock rolls.
Joe
What is that grade? And is the grade 50 cars long or is only part of the train on the grade?
On a prototypical 2.2% mountain grade with the entire train on a grade I think you are going to have a hard time finding any six axle diesels that will handle a 50 car train with cars weighing anything close to the prototype or even to NMRA recommended practice.
On model railroads it is pretty common to push grades a little beyond 2.2% with 3% being the end of what most would agree is a reasonable maximum. That doesn't mean that grades over 2.2% are desirable but there are reasons that cause people to do it. Those reasons may include gaining height in a limited distance for a flyover or in a helix. There is a big performance penalty to going to 2.2% grades or over, especially without traction tires. Marc may need to give that some more thought depending on how he would like to design his layout.
I'm not sure how many cars a real engine handles.
For a contemporary 110 car unit train with Dash 9s, ES-44s or SD70s expect to see 3 units on an empty and 7 units to get a loaded train over a 2.2% grade. Without traction tires I expect that you would need more than three units on an empty and more than 7 units for a train at loaded weight to get an O scale train over a 2.2% grade.
I have a couple of steamers in two rail. They slip more than the diesels. My Pacific won't handle too many cars on it's own.
Steam is another consideration for Marc. Again it will depend on the design of his layout and how he wants his steam power to fit into his operations. It is harder to generalize about steam than one manufacturers 4 axle or 6 axle diesels. Some brass steam passenger locomotives have a hard time pulling a prototypical consist even with traction tires on level track. Some steam locomotives are great pullers.
I think guys who run two rail are more interested in modeling after the real thing.
Stereotypes come from somewhere. But I think the changes in the hobby have made some of those stereotypes much less true than they used to be or just downright blown them up.
Today's three rail isn't just about 1950s 6464 box cars and 2 rail isn't just about brush painted cars built from 1950s kits either. Both segments of the hobby have come a long way thanks to the mountains of money that have been spent on O scale trains and the investment in new tooling that was made to serve and expand that market.
The investment by the manufactures has given us great benefits. It is much less expensive to model realistically in 2 rail or 3 rail O than it was 20 years ago. And it can be done in much less time, or at least with much more ability to chose where you would like to spend your time. Scale model railroading with 3 rails still has space advantages over 2 rail. Three rail has cost and time advantages too if prototypical signaling is important. Two rail has, well, two rails. Benchwork, scenery, structures, room lighting, control and rolling stock will be similar if not identical challenges.
Even though the design thus far is an island type layout,
Marc
STOP!
Don't do it!!!
Island type layouts are popular thanks to the existence of 4x8 sheets of plywood and ping pong tables. That doesn't make them desirable as model railroads nor does that make them efficient users of the space in your home. Upsizing island layouts makes them more problematic in any scale. Scaling up an Island layout to the width needed for continuous running on 2 rail O will leave you with either unworkable reach in problems, undesirable duckunders or massive wasted square footage.
Along with your considerations of the relative merits of 2 rail and 3 rail O take a look at other layout concepts for you layout space. By going with something like a folded dog bone, an around the walls or an around the walls with a center peninsula concept you can have broader curves, more space for the railroad, more space for people or all three.