For my 027 sharp curve layout with 27 Marx switches the best conventional engine is hands down the Lionel 2035.
I started out with light weight 242, then 2026 and then 2025 (675) and then found the magnatraction 2035.
This is the best pulling, reasonable cost, stay on the tracks loco I have. It will pull 6 or 7 heavy operating cars like the milk and cattle car which other locos have trouble getting to move.
I will admit the 2035 is not the best looking loco, being sort of short and stocky looking, but its operation makes up for lack of looks.
I have 5 or 6 Lionel 2035 and usually have 4 on the layout most times. Others like the 2056 etc have trouble with the curves and getting started.
Charlie