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? Thank you.

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