Hello! Welcome to the forums and Simutrans. I am glad that you are finding the game interesting. I answer your questions under subheadings below.Base profit per km
Because the revenue in Simutrans-Experimental varies depending on the speed, and, for passengers, the comfort, and because the extent to which each of those two things affect the revenue is in turn affected by the distance travelled or journey time (passengers are less willing to pay extra for luxury on a 5 minute 'bus ride than a 4 hour flight, for example), it is not possible to give a single total figure for profit. The base profit figure is therefore what the vehicle would earn if it were travelling at what is calculated to be a neutral speed and if (in so far as passengers are concerned) it had the tolerable comfort level for the distance in question.Tractive effort and power of locomotives
There is not a tutorial for calculating to which tasks that any given locomotive is best suited. However, the physical parameters and performance of the locomotives are as closely based on their real life counterparts as possible. You might thus find it helpful to use Wikipedia to look up what they were actually used for.
In very general terms, power determines the weight that a locomotive can haul at any given speed and tractive effort determines how long that it takes to accelerate to that speed (and whether it can start at all with any given load). The exact relationship between power and tractive effort, however, is different for steam locomotives than other types of locomotives because steam locomotives are constant force machines whereas others are constant power machines. See here
for more information on the difference.Distance and cost of building ways
Distance is indeed 125 meters per tile at default settings. However, the cost of building ways is made up of three separate components: (1) the cost of the land; (2) the forge cost (i.e., the cost of building the track or road bed, minor earthworks, drainage, etc.); and (3) the cost of laying the way itself. When building a new way, all three costs are payable (unless you already own the land). When upgrading one way to another of the same type, only the last cost is payable. This last cost is the cost shown in the toolstrip.
As to corners, each tile of diagonal way is deemed to be 1.414 times as long as each tile of straight way, and travelling time and costs computed accordingly.