I don't know if Lego designed the locomotive smaller for cost purposes or if they made it smaller to run on Lego track, I think it's unclear. The best train "models" are well balanced between motive power and rolling stock. In the catalog image, the whole thing just doesn't look right from a train perspective. It includes two coaches of significant length compared to the engine. In the Lego world we imagine we are pulling more cars when we run the train, but honestly that engine looks more like a switch engine than one that could make a transcontinental trip pulling a string of coaches. It's a 4-6-0 configuration. In my book it's definitely missing a trailing wheel/truck. They could have gone with a 2-6-2 configuration or 2-6-4, and it would probably have looked more balanced and like it could pull a long string of cars. I don't think they needed to do a x-8-x configuration - that would only be possible if the wheel diameter were really small or if it did no run on Lego track, but pilot and trailing truck on a transcontinental locomotive would see like a given. However, I'm in the USA, and am not very familiar with European/Asian steam locomotives.
Since real train offerings from Lego are pretty scarce, we train fanatics will buy anything on offer, and if we are smart (and can afford it) we buy at least two of whatever is offered (I should have bought two Emerald Night, two Maersk, etc, etc.) I could see buying two copies of this set ($600, ouch!) to get four coaches and then maybe "MOCing" the engine to something more suitable. I have no idea if that would work, or if I personally could even attempt such a thing. I'm not very good at coming up with something completely new, but it's fun to try. @samsz_3 in your lengthened picture, would that require a whole additional section of the boiler, or are you just lengthening the wheel base? Are parts readily available in these colors to extend or modify the engine without needing to buy a second set?