Locomotive Wagon Capacity

[=7CAV=SPC.Henson.K]

How can I judge the strength of a locomotive's ability to pull wagons? How long can I make these trains?

[Metacritical]

this is only really relevant at the start of the game when trains are slow and weak, later on they are fast and strong and are limited largely by the length of the platforms (though these can be ignored).

for passenger trains on European maps i have four to six wagons and if i need more i add an extra train. for cargo i usually have a single train with one wagon to start with as this keeps the costs down before the line starts producing stuff. then i auto replace to add five or six wagons and wait to see how profitable it is, though in the early game you are more limited by the demand from cities rather than the amount of stuff your cargo trains can carry. several faster smaller trains are normally better than a one or two slower longer trains.

[Mr Felidae]

Its a bit of a guessing game in my opinion. Only when having added wagons to a locomotive will you get a power rating. Only the "Good" and "Excellent" ratings are preferable. You may always add a second locomotive to the train, though this, of course, increases maintenance costs significantly.
Train length really depends mostly on the infrastructure of the route your train is going to use. It's always advised to have route sections (distance between two signals servicing the same direction of travel) to be no shorter than the longest train running on that route. For an exact answer on maximum train length someone with that knowledge will need to answer this question. Personally I believe the game mechanics don't have a limiter on train length, but your balance sheet certainly does...

[Nick B.[US]]

I mainly play with the no costs mod so I buy a loco I think is good and then start putting rolling stock on it and see what it can pull. If I need more power I either choose a different loco or MU more than one on a consist (train).

[Stealthy]

My rule of thumb which has served me well (never had any financial issues in the game on any difficulty level) is: Maximum train weight = 2 x kN of the locomotive. In other words, maximum weight of the wagons added based on the info can not exceed 2 x pulling power of the locomotive.

Locomotive with 90kN pulling power can pull max 180 tonnes. On fairly level ground. Steep uphills and you need to cut that back a lot.

If you need more, double heading or even triple heading is an option even at early game. Buy a locomotive, buy another. Move the second one up to first train and voila, you got 2 engines meaning 2 x pulling power and potentially 2 x train weight & length.