Indeed, and add that in Tunnels the max speed is 450km/h! almost as fast as a Maglev!!!
But then the game would be more boring. The constant fear of molamen who can attack you at any part of the 3 big tunnels, or to get free Mammoths or cheap antiques from the nomads...

This is actual a very complex problem in the concept!
WHY is the train going slow to fight another train? You could just run away, no?
The only idea I have is that you have after some hundret kilometres a area in which there are two new tracks which are 'fighting tracks' and on which you can't drive faster than 50km/h - else you will derail.

As for the mole people or other encounters:
You need to be able to stop a train for static encounters.
How do you enforce such a train to stop?
If you just damage the tracks it will derail, of course.
If you damage it and the train can 'sense' it (through the use of some device),
then you can actually force the train to stop, send out guys to repair the tracks and go on.

The Transarctica is faster than the Viking Union trains so there have to be a plausible cause to enter a fight. Maybe the Viking Union trains have a secret capability which was refitted after the Transarctica had been stolen. I know that some german U-Boats got some sort of turbocharger which was powered by exhaust gasses of their diesel engines so that they could achive a faster surface speed.

Maybe the Viking Union trains could "close" their funnels to give some extra pressure to the cylinders and gaining some more speed once they've spotted you.

Transarctica was one of kind locomotive we stole and at top speed you can escape the Viking union trains. (Other one is the last Locomotive train the Minotaur....)
Maybe there was a concept behind them to get information on cities, places and other trains and so on like secret documents, map charts or train schedule papers which later was not included. Also if you fight a freight train which only have MG wagons or a single Cannon one, you hit the jackpot as these trains usually carry high valuable cargo!

Also you are a person who against the Viking Union and you stole their most advanced most precious train so they will hunt you down.
It is like a war, you cannot won if you never fights!

Another wagons I've thought of:
Medical Wagon and XL Medical Wagon "Hospital": Can carry sick or injured people between towns, also helps healing injured train personnel and soldiers. Slightly improves the medical facilities of other wagons like the barracks, spy, command etc.
Would look like a barracks or XL barracks wagon with less windows, white as the refrigerator and the well known red cross on the side

Medical Wagon upgrades:
Better Medical Equipment: Costly equipment but improves the patients recovery rate
Effect: +X% faster healing rate (less day to be fully healed)
More Patient Rooms: Rebuild the interior of the wagons so more sick people can be carried but less room for the surgical and other medical spaces meaning longer recovery rate.
Effect: +X capacity, X% less healing efficiency
More Medical, Surgical Rooms: Rebuild the interior of the wagons so more rooms for the nurses, doctors to work on people but would carry less sick people.
Effect: -X capacity, X% more healing efficiency
Light Armour Plates: Defence against Machine Gun Shells.
Effect: +X tons of weight, +HP
Heavy Armour Plates: Defence against cannon shells
Effect: +X tons of weight, +Armour
Partial Reconstruction:: Replace parts of the wagon's elements with lighter materials like Wood, Copper, Bronze, Aluminium
Effect: -X tons of weight, -HP
Complete Reconstruction: Entire wagon is replaced with lighter materials but it effects the wagon's usefulness as it was structurally weakened
Effect: -X tons of weight, -HP

So the idea would be to have next to the 'normal' passengers also sick persons who want to get transported to a town with a good hospital?
Not bad!

Maybe we can have the luxury-hotel - version of a wagon too?
For those who want to travel in style... and in a 200 qm² appartment with full-service!

I'm not so sure.
If the Viking Union is the ruling company, then the few rich people would have connection with them, and those we sure not want to carry.

Based on the wagon data I've collected here:
http://forums.sunlightsystems.de/viewtopic.php?f=12&t=571
and based on axle loads from here:
http://en.wikipedia.org/wiki/Axle_load#Locomotives

I've made some calculations for the revised wagons.
Base axle load: 20tons (metric)

Normal Merchandise Wagon:
Empty weight: 45tons
Standard weight: 50tons includes 5tons of supplies and equipment
Maximum weight: 120tons
Cargo capacity: 70tons
Wheels: 3-3 axes

XL Merchandise Wagon:
Empty weight: 55tons
Standard weight: 60tons includes 5tons of supplies and equipment
Maximum weight: 120tons
Cargo capacity: 60tons
Wheels: 3-3 axes

Note for above: I doubt we will distinguish carried goods by sizes as the XL surely could carry less in weight but bigger in size I suggest set cargo capacity to 40 for standard and 60 for XL

Barracks Wagon:
Empty weight: 80tons
Standard weight: 110tons includes 0.5tons of supplies and equipment per each soldier and extra 5tons for wagon supplies and equipment
Maximum weight: 160tons
Cargo capacity: 50 Soldiers /25tons
Wheels: 4-4 axes

XL Barracks Wagon:
Empty weight: 100tons
Standard weight: 145tons includes 0.5tons of supplies and equipment per each soldier and extra 5tons for wagon supplies and equipment
Maximum weight: 160tons
Cargo capacity: 80 Soldiers /40tons
Wheels: 4-4 axes

Tender Wagon:
Empty weight: 50tons
Standard weight: 55tons includes 5tons of supplies and equipment
Maximum weight: 120tons
Cargo capacity: 650 Bak /65tons
Alternative:
Axle load: 25tons
Standard weight: 55tons includes 5tons of supplies and equipment
Maximum weight: 150tons
Cargo capacity: 950 Bak /95tons
Alternative:
Axle load: 40tons = Real world record/maximum
Standard weight: 55tons includes 5tons of supplies and equipment
Maximum weight: 240tons
Cargo capacity: 1850 Bak /185tons
Alternative:
Axle load: 95tons = Possible maximum on these rails???
Standard weight: 55tons includes 5tons of supplies and equipment
Maximum weight: 600tons
Cargo capacity: 5150 Bak /515tons
Wheels: 3-3 axes

Note for above: I've used these various calculations to see what is needed for an adequate coal supply

Cannon Wagon:
Empty weight: 100tons
Standard weight: 145tons includes 40tons of supplies and equipment for the weaponry and extra 5tons for wagon supplies and equipment
Maximum weight: 160tons
Wheels: 4-4 axes

Note for above: Need to determine cannon calibre as it will determine shell size and weights

Machine Gun Wagon:
Empty weight: 40tons
Standard weight: 60tons includes 15tons of supplies and equipment for the weaponry and extra 5tons for wagon supplies and equipment
Maximum weight: 120tons
Wheels: 3-3 axes

Prison Wagon:
Empty weight: 65tons
Standard weight: 130tons includes 0.5tons of supplies and equipment per each slave and extra 5tons for wagon supplies and equipment
Maximum weight: 160tons
Cargo capacity: 60 Slaves/30tons
Wheels: 4-4 axes

Alcatraz Wagon:
Empty weight: 85tons
Standard weight: 140tons includes 0.5tons of supplies and equipment per each slave and extra 5tons for wagon supplies and equipment
Maximum weight: 160tons
Cargo capacity: 100 Slaves/50tons
Wheels: 4-4 axes

Livestock Wagon:
Empty weight: 60tons
Standard weight: 100tons includes 20tons for the mammoths themselves, 15tons of supplies and equipment for them and extra 5tons for wagon supplies and equipment
Maximum weight: 120tons
Wheels: 3-3 axes

Harpoon Wagon:
Empty weight: 20tons
Standard weight: 40tons includes 15tons for 3 extra harpoons and other equipment and extra 5tons for wagon supplies and equipment
Maximum weight: 120tons
Wheels: 3-3 axes

Refrigerator Wagon:
Empty weight: 90tons
Standard weight: 100tons includes 10tons of supplies and equipment
Maximum weight: 160tons
Cargo capacity: 60tons
Wheels: 4-4 axes

Bio-Greenhouse Wagon:
Empty weight: 40tons
Standard weight: 55-60tons includes 15-20tons of supplies and equipment
Maximum weight: 160tons
Cargo capacity: 100tons
Wheels: 4-4 axes

Note for above: similarly to the merchandise wagon issue I suggest the cargo capacity to reduced to 30-40tons

Tanker Wagon:
Empty weight: 40tons
Standard weight: 45tons includes 5tons for wagon supplies and equipment
Maximum weight: 120tons
Cargo Capacity: 500,000 Litres
Wheels: 3-3 axes

Oil Tanker Wagon:
Empty weight: 45tons
Standard weight: 50tons includes 5tons for wagon supplies and equipment
Maximum weight: 120tons
Cargo Capacity: 500,000 Litres
Wheels: 3-3 axes

Note for above: Need to calculate maximum liquid capacity for these tankers but I suggest around 500,000 litres

Observation Box Wagon:
Empty weight: 100tons
Standard weight: 110tons includes 10tons for wagon supplies and equipment
Maximum weight: 120tons
Wheels: 3-3 axes

Observatory Wagon:
Empty weight: 110tons
Standard weight: 120tons includes 10tons for wagon supplies and equipment
Maximum weight: 120tons
Wheels: 3-3 axes

Spy Wagon:
Empty weight: 50tons
Standard weight: 70tons includes 10 tons for the spy planes and associated equipments, 5tons for spy equipment and extra 5tons for wagon supplies and equipment
Maximum weight: 160tons
Cargo capacity: 5spies/5tons
Wheels: 4-4 axes

Missile Launcher Wagon:
Empty weight: 120tons
Axle load: 120tons
Standard weight: 135tons includes 15tons of supplies and equipment
Maximum weight: 150tons
Rocket weight: 5-10-20tons?
Wheels: 3-3 axes

Boiler Wagon:
Axle load: 200tons
Standard weight: 220tons includes 20tons of supplies and equipment
Maximum weight: 240tons
Wheels: 4-6-2 axes

Crane Wagon:
Empty weight: 55tons
Standard weight: 70tons includes 15tons for wagon supplies and equipment
Maximum weight: 120tons
Wheels: 3-3 axes

Drill Wagon:
Empty weight: 105tons
Standard weight: 120tons includes 15tons for wagon supplies and equipment
Maximum weight: 120tons
Wheels: 3-3 axes

Still need to get info on these: (Any help appreciated!!!)

Locomotive:
Axle load: 95tons = Possible maximum on these rails???
Weight: 1000tons (Standard?)
Maximum weight: 1045tons With ingame variant
Maximum weight: 475tons With intro screen variant
Wheels: 4-6-4-4-4-0 or 4-4-2-0 wheel arrangement

General Quarters Wagon
Empty weight: 50tons
Standard weight: 60tons includes 10tons for wagon supplies and equipment
Maximum weight: 120tons
Wheels: 3-3 axes

Boudoir Wagon
Empty weight: 40tons
Standard weight: 50tons includes 10tons for wagon supplies and equipment
Maximum weight: 160tons
Wheels: 4-4 axes

Just from the number I would say that max axle load is 100 tons.
Does this makes sense for the other numbers too?

I'm not sure, what other numbers are you referring to?