Hmm, less than a minute in the video, and they're already confusing % and °, wondering about the quality of the rest of it ?
"The tractive force has to overcome the weight of the train"... kind of misleading too, as on NO incline, IIRC the train only has to overcome static and/or dynamic frictions (which for the wheels/rails contact might or might not be related much to the downward weight force, I guess larger weight means somewhat more contact surface for friction, and actually better performance ? P.S.: Ah, yeah, he talks about that later).
The gradient doesn't "add to the drag", it's just that the train now has to overcome gravity too. And since gravity now won't be perpendicular to the rails, I guess that friction is going to decrease, making the train operate even worse ?
It does not change the underlying truth: Trains suck at gradients. Either direction.
The one thing that gives them their efficiency, is the low drag from running steel wheels on steel rails.
A lot less then rubber tires on asphalt. They are a bit like a Ice-Skater, pulling a sled. The only thing more efficient, is a ship on the sea.
But that same property also makes them unable to deal with even weak inclines. If you are going up and try to go to fast, you end up with a train spinning it's wheel. If you go downhill and try to fast, you end up with a runaway train (basically a bit like a car suffering from Aquaplaning while going downhill).
Their acceleration/deceleration ways are measured in the hundreds of meters.
Once you realize the rules, you can see it in the design/useage of every single train track.
Look at the Appalachian Mountains, the Rocky Mountains, the Alps - all giant obstacles for Railways, in a way they never were for cars.
If you ever saw a "railway" going up a steep incline without issues, it was propably a Rack/Cog Railway. Those have been used in that time between railway and cars. But now are only kept for posterity reasons.
I have a Cog Railway, a steep incline/curve combination and very long stretches to go underground in my local Area train network. I can see the issues every time I want to.
If you hit a hill, it is easier to go around the hill in a wide turn or through the hill with a tunnel - rather then trying to go uphill/downhill with a train. That explains the giant pricetag for building in mountains.