I will try to help you. Below are pictures I took of my 2 Lionel scale Big Boys.
Lionel Legacy #4012, (6-11126) vs Lionel #4006, (6-28029)
In ALL photos, the Legacy version is closest/bottom engine in the pictures!
(ABOVE, Legacy on RIGHT of photo) First, I 'squared' the angle iron to the edge of the table. Then put the track against it, so I have an equal starting point for measurements and perpendicular track.
(ABOVE) Next, I put the locomotives on the track. I gently pushed the 'cow catcher' each engine against the angle iron so no damage occurred. Again, Legacy is in the front. Ensured there was no 'play' in the engine / tender.
(ABOVE). This photo was taken square to the tape measure. The Legacy one is right about 34" as measured at the coupler.
(ABOVE, Legacy front) The TMCC version is just a bit over 35". I then turned each Big Boy over and examined them. There doesn't appear to be any way to change the draw bar length on either version. Mine are fixed. Slack in the engine / tender draw bar isn't enough to account for the different lengths. Besides, I ensured there was no slack before photos.
(ABOVE, Legacy on bottom) Here are the two Big Boys side by side. As you can see, there is quite a bit difference in length. Why?
(ABOVE, Legacy bottom). The tender body lengths are exactly the same. Not shown here, but I measured them.
(ABOVE, Legacy bottom). Look at the difference here! The Legacy version is much 'closer coupled' than the TMCC version. Both Big Boys are correctly attached to the tenders and even with the angle iron. Pictures are worth a 1000 words!
(ABOVE, Legacy bottom). I removed the tenders again, and this caught my eye! Anyone notice something? Look how much shorter the Legacy cab is versus the TMCC version.
(ABOVE, Legacy front). Another picture showing the difference in actual engine lengths.
(ABOVE, Legacy on RIGHT). Look at the back of the cab edges, the cow catchers are even in the picture. Also, notice how the Legacy version everything (smoke stacks, whistle, etc) is closer to the bottom of the picture. Part of this is the photo, but there is a noticeable difference in item locations on the two different Big Boys.
I bought each Big Boy for different reasons. I'm not a 'rivet counter' but I do have a preference for scale engines. Too me, they both look great. The 'close coupled' look of the Legacy version is nice. Which one is best? I don't know, depends on what you prefer. Each engine was top of the line when it was introduced. The Legacy is much better detailed overall, which is too be expected.
I would say the following if you are looking for a Big Boy: If you are a die-hard train buff and can't afford the new Vision Line Big Boy, the Legacy version is great. It'll do anything you want. If you can't afford the older Legacy BB, the TMCC version would allow you to have a lot of fun and play value. It is acceptably detailed and sounds decent. Careful hunting can reward you with deals that are well below $1,000. I've saw them from $500-$900 frequently, depending on condition.
I don't think you can go wrong with any Big Boy. They're just awesome. If you are a Union Pacific steam fan, you need to have at least one!
I'll have to update this thread when my Vision Line Big Boy arrives. It would be awesome to see all 3 compared. HHHMMM, I have an idea. Hope you all enjoy!