Well, the K-Line will be significantly smaller than the RailKing. I have the K-Line Allegheny, and it's really somewhere between O-scale and S-scale, probably about 1:58. The Railking is 29" long vs 33" long for the scale big boy.

 

I also have the K-Line Allegheny.. It's a nice engine.

 

This eBay ad has a picture of the MTH Big Boy box. And it states O31 curves and supports John's posted length.

 

I have the K-Line Allegheny, it was one from way back that I upgraded. It is small, but it's nicely proportioned and looks better than some of the other semi-scale stuff I've seen.
That one is a conventional model, kinda' expensive for a 20 year old semi-scale BB.

 

One thing to note, I believe the K-Line big boy, although very small as mentioned, is shrunken a bit more proportionally in all directions than the MTH one. I think the MTH one is a bit closer to scale in some dimensions and shrunken in others which has always struck me as being pretty awkward looking. In my eye the K-Line unit looks a lot better overall including some details the MTH one doesn't have (perhaps newer ones do?). The one downside to the K-Line one is that the running gear is missing all the valve gear linkages and only has the main and side rods present.

 

I've had both, plus the Lionel Lionmaster. I would recommend it, alot over the two others, but then the MTH. The K-line is too small for me.

 

When I was running a hobby shop I had many pre-orders for K-Line Big Boys after their announcement. After they arrived many customers didn't tale them because they were too small. The guys with smaller layouts didn't order any but loved them. I didn't have any trouble selling them to those guys.
The K-line Big Boy is small and has a simplified valve dear. If you have the curves and clearances, buy the MTH.

 

Get a Lionel Lion Master Big Boy.I have one and it will run on 042 curves. Very detailed, with all appropriate sounds.

 

I have the K Line big boy Lionel released for S gauge. It uses the same castings for the engine and tender. It is 25” long making the length just about 1/64. The height and width are about 1/60th. Here is a picture of the Big Boy on the left and a true 1/64 scale Challenger on the right. The Big Boy fits in pretty well in the S gauge environment, it would look way too small among 1/48th equipment. The S gauge version will handle 20”radius curves, I use minimum 30” radius and it works well. I cannot imagine this engine on O-31 curves.

 

You're missing a taillight lens.

 

Sure enough! It is amazing what becomes visible in a picture that the eye misses when running the trains. I posted some detail pictures of cars on my layout in another thread and quickly saw three minor things on the layout that needed repaired. Never noticed them just walking around the layout.

 

It happens I notice that because one of my recent purchases is also missing that lens. I have it on order from Lionel, but who knows how long that'll take.

 

I got an RA for a new SD70ACe that was inop. They received it the day before Lionel shut down. I suspect it will be some time until I see that engine again.
On the big boy tender note the rear axle has flangeless wheels. As delivered those were flanged but the flanged set had to be moved forward on the tender and swapped with the flangeless wheels to allow it to back up w/o derailing on 20"R curves.