We will first look at the top of the model (Train2).

It looks like you have made one half and then mirror, but at the top seam/join the surface dips in, it does not make a smooth surface. This on its own will not cause a major problem for the shelling, but if you attempt a fillet along the edge then it will have problems. (it would also show up in a render, which is not always wanted)



What I did to check this was to select the model, click on the 2 end edges, then copy - paste (which will copy the edges and then paste back the curves), you can then select "show points"



In the right hand view you can better see the control points that make up the curve. The center point is actually 2 points, one on each end of the 2 curves, now as you move out on ether side of those center points you can see the next control points are higher than the points in the center, this makes the curves dip in (if you select those 2 points and move them up or down (in the right hand view) you will see this dipping more clearly.



So when creating those curves for the surface, make sure those 2 points line up with the center points, you can use the "Align". Just select the center points and those 2 points on each side, then select "Align" - "Horiz center", then click on the center point which will align those point horizontally. You can then use the curves to extrude to create the surface




When I edited your model, I actually trimmed off the top surfaces and used blend, but it is better to get the curves aligned correctly then no editing is needed.

If anything is unclear, then just ask.


(I will make a further post a little later, I need to go out now)