Fast shooting lens for Nikon D7200?

Hi I'm upgrading from a D5100 to a D7200, I've been photographing for about 5 years. I would say my experience is intermediate, although I don't know a whole lot about lenses. I really only take photos of trains, this requires a lot out of a lens. I've been using the Nikkor 18-140mm lens for a few years now and I notice it doesn't focus as fast as I need it too. The way I usually shoot trains going by is I zoom in to around 50mm or so and as the train comes closer I zoom out with the train keeping it in the frame till it passes. This requires me to constanly keep pressing the shutter to "re-focus" the lens so it can keep the train in focus as I zoom out wider as it comes closer. With the current setup I have now (D5100+that lens) when I'm doing that sometime it just does not focus in time and I miss the shot. I've missed many of nice shots because of this. Among other upgrades the one I want most with the D7200 when i get it in a few weeks is quicker focusing. Will this be greatly improved with the D7200? Or is it the lens? Advice?

You need to learn a technique used by All sports and action photographers.

There's a button on the back of the cameras that when pressed will follow focus as you shoot any action or sports. It is the AE-L/AF-L button. In your cameras menu, assign that button to auto-focus.

Using this button will release you from pressing the shutter release part way down as you follow the action

The lenses you have will certainly do the job because they are the Nikkor fast focus lenses, AF-S and are very capable of focusing fast. I use them all the time when shooting football, basketball, motocross and drag racing.

Take advice from Fhotoace.

When I come across questions like this, it makes me ask myself just exactly how photographers went about their business before 'autofocus' came along… I mean there wasn't a shortage of quality images in publications in the 1960's 1970's or 1980's.

You could practice pre focussing your lens and wait patiently for your moment, it's a decent technique to learn.