Whats going on in this image

This is straight from Camera. EOS80D - EF-s 18-135 IS STM 1/160 @ F6.3 ISO160
According to the maths on the various online calculators, because the point that grabbed the autofocus was the cylinder gear on the loco (just above the front wheels)
the settings used meant the hyperfocal distance had been exceeded, everything in the shot should have been in focus.
Clearly the people in the background are well out are is the trees.
even the number on the cab door is starting to lose sharpness.
AE failed and overexposed the shot. A lot was recovered by DXO optics pro, but the detail in the white ballast couldn't be got back.
The camera has blown the greens a bit but this seems to be a known issue with the 80D landscape setting.

On 100% crop the leaves on the distant tress have faded at their tips to grey and narrow twigs are non existent.

Is this CA in the lens, or blooming on the sensor, or down to overexposing, or am I expecting too much from the EF-S 18-135mm STM lens. (I'm used to working L series)

The previous weeks shoot using my 6D and 35mm f2 IS USM achieved better depth of field and detail in the tiny twigs on the periphery of the bushes was maintained

The only thing I can think of is panning. By the numbers, the entire photo should be acceptably sharp. It clearly is not.

Considering that it was overexposed it looks OK to me. (There's a similar shot on the Worth Valley website)

How fast this thing is moving? I would take it at least with 1/500 to have sharpness. Not 1/160.
AE is not something to trust if shot is important. Better to take test shot and adjust if it is enough time.

Without knowing the settings you used with the 6D & seeing an image, & not knowing if you were panning & how good you are at it, it's not a terrible image considering that you let the 80D (mis)manage the exposure & focus point. Then there's the in camera processing which can lose a lot of detail if you didn't start with a RAW image.

low shutter speed + wind blowing the leaves?

Was the OP using a single selected focus point? Or letting the camera pick? I get the feeling from looking at the picture as if the AF has grabbed on the rails in front of the loco. There is probably enough DoF that the AF would also light up on an AF point centered on the valve gear. This would bring the focus distance much closer, probably to before the HFD and so I would not be surprised at the drop off in focus in the far distance. Of course if the loco is also moving, and the OP is also having to pan with it that will also affect the stationary background. Given he is at 1/160 I don't think it could have been moving that quickly.

Alan

The train was at walking pace dong the 'run around'. Usual train practice seems to be 1/100th for every 10mph although experience has shown they can blur at amazingly low speeds, some also may be due to vibrating panels on the loco's. Much as it's the reviewers main crit of the 6D that it only has a few focus points I'm quite happy with a single centre one most of the time, and I may go back to this on the 80D more. The image here was shot using the 9 centre points which could have pulled the focus forward a bit. I do all my post processing in DXO which has pulled the 80D a lot for overexposing and most of my images need pulling down by about 1/4 stop. The 18-135 is sharp but not tack at 135 on long distances. The landscape setting is an issue Canon need to address as it's blowing the greens to shreds. Seldom have I had to desaturate a colour in a shot. The new sensor is a vast improvement on the 70D and some say better than the 7Dii with a much better DR. If Canon haven't adjusted the settings in the firmware then this may be why its doing it.
Thanks for your thoughts and input guys