Both grade and curve figure into tractive effort required. With grades you're fighting gravity, and with curves you're fighting wheel resistance. The sharper the curve, the more the sides of the wheels are going to rub against the rail. Combine the two and you're just making life more difficult for train operations. Speed on the curve is also limited by centrifugal force. Go too fast and the train will tip over (can we say Jenkins Curve?). From my home northward the rail line essentially runs downhill, along with the river going downstream, yet pushers were required in steam days not for the grades along the way, but for all the wheel resistance on the curves, of which there were numerous. When they ran unit coal trains from a large mine just to the south of me to the eastern part of Pennsylvania, every so often they had to turn the entire train just to equalize wheel wear on the cars. Apparently there were more curves in one direction than the other, or the curves going in one direction were more severe than those going in the other direction. One normally would not notice such wheel wear on go-anywhere freight cars, except these were the same coal cars going over the same route, day after day, the it showed - the wheels on one side became more worn than those on the other side. Turning all the cars on the wye solved the problem.
Regarding knowing the weight of the train and the grades en route, most railroads, when they received new engines, came out with tonnage charts showing the maximum tonnage allowed for one unit over selected portions of the line's route, and eastward ratings might well be different from westward ratings. If more than one engine, tonnage ratings were cumulative. For the New Haven, such ratings were shown in their Arranged Freight Service Symbol Books or in a separate publication.
For example, from Symbol Book No. 20, the following ratings applied for the Cedar Hill - Bay Ridge route:
Cedar Hill to Oak Point: 2,500 tons, both westward and eastward for all diesel-electric road engines except for the DERS-7s and DERS-8s. For them it was 2,900 tons westward and 3,100 tons eastward. For the EF-4s, it was 3,800 tons westward and 4,250 tons eastward.
Oak Point to Bay Ridge: 1,300 tons westward and 2,100 tons eastward for all diesel-electric road engines except for the DERS-7s and DERS-8s. For them it was 1,700 tons westward and 2,600 tons eastward. For the EF-4s, it was 2,300 tons westward and 3,500 tons eastward.
Clearly, the EF-4s could outpull anything else on the road.
There was a special note for the newer DERS-4/5/6s limiting train tonnage to 4,500 tons westward between Oak Point and Bay Ridge when three units were used.
Tonnage ratings were not included in Symbol Book No. 14, but seeing as how 14 and 20 are the only ones I have, I can't say when they started showing such in those books.