If I’m reading your question correctly, you almost have a 6% grade which is almost double a 3% grade which is generally regarded as the absolute maximum grade in prototype & model railroads. If the track run you describe is also going around a curve, the work the engines have to do is even greater because of the operational friction a curve generates. If you’re pulling a six car train with one engine, you’re doing pretty well all considered. Realize that it IS okay to run a double or triple header engine configuration with the conditions you mention. That’s what actual railroads do to get a load over the ‘hump’.

Hope this helps,

Geof from the US