Hmm, using this, we can calculate both Little Strongheart and the train’s speed.  
First, we can observe that LS gallops over two wagons in roughly 1.2 seconds, giving her a speed of 1.66 wagons/second. Now, size relations are, of course, heavily debated, so it depends on which standard you adhere to, but for my calculation here, I’ll assume that LS stands about 1m high (top of head), meaning that carriage is roughly 4-5m long. If we take 4.5m, LS’s speed in relation to the train is 7.5m/s, or 27km/h.  
In addition, due to the mountain-image above, we can split up this speed into the speed by which the train moves right, and the one by which LS moves left. Yes, LS is actually moving left in relation to the ground.  
By measuring the apparent length of a wagon in the two images, we can conclude that LS speed over ground and the train’s speed over ground are roughly 1:2.8, and if we now split up the total speed accordingly, we can conclude that LS moves left at about 7km/h in relation to the ground (27km/h to the train), while the train’s speed is ~20km/h.

SCIENCE FIGHT!

@jspenguin  
Nope again.  
On the train, her speed relative to the ground, so relative to the wind too, would be higher. So the friction would be too.

Cool. Maybe it will work.

@BigMax  
Actually, the air would be helping her on the train.

@Gomenasai  
Eeeeenope. Galilean relativity.  
She would be as fast on the ground.
 
Or even faster, if you consider the air friction on the train.

OP, you’re a genius

Sweet.  
I suppose she appears to run faster due to the train moving backwards under her feet, thus she’d run slower on still ground. But this is cool.

This is neat, I like!

This is really cool!

@Teraku  
I am well aware of parallax. Your point being?

There’s this thing called parallax. You should look into it.

Of course, the mountain is going to appear to move less than the train due to parallax.

neat! thats some nice gifwork right there

Nice observation.

Hey cool. She’s pretty fast. Cant’ wait to see the result.