In the 19th century especially, the locomotives available can't haul much and still travel at their rated speed. In real life, heavy freights and fast express trains were double headed - two locomotives were hitched together to give double power. Also, horse-drawn trams can only be 1 car long, ore else even empty they will not exceed 10 Kilometers/hour. Could there be multiple teams pulling these? This would make running large systems much easier.
This can be done already. Just put two locomotives behind each other.
Look at this page: http://www.akrangard.cz/simutrans (http://www.akrangard.cz/simutrans) (year 1891) to see trains with multiple engines.
(http://www.akrangard.cz/simutrans/pic/ed2x2.png)
However, except for some extreme cases, I found that two short trains provide a better service rating at stations than a single long train. Of course this is sometimes balanced by the need to plan for a place where one train can pass the other, but in my experience it's still more effective.
If you need double track anyway, many shorter trains are generally better as the station size required is smaller and therefore cheaper. Only when your network is jammed with too many trains do you need longer ones...
My bad!