Using the Lathe Modifier
In this example I will be using the lathe modifier on a spline to make the rounded end of a observation car.
The first thing I need is a spline object. I make this by selecting the edges that makeup the end of the car I want to round.
Once I have these selected I click on the Create Shape from Edges button.

Now I have a spline to work with. I select the shape and save the spline ( File > Save Selected ) to its own file
so I can use it again else where when needed.

Next with the spline selected, I go to Point Select mode and delete the points that are to one side of the center.
If you do not have a point, top and bottom at 0 on the X axis, you need to insert them in this mode with the Insert command
under the Geometry section. Just click on the Insert button, then click on the spline and drag it a bit to imbed the point.
Turn off the Insert button and position the new point so it is at 0 on the X axis. Be sure it is also positioned on the Y axis as the
other points are and aligned vertically.

It should look something like this when your done.
Now leave the point edit mode and in your modifiers dropdown select the Lathe modifier.

Set the number in the Degrees box to 180.0
Check off the Weld Core box.
Set the number of Segments to what ever you want. This affects the number of faces you end up with.
Keep is as low as possible but still high enough to give a smooth shape.
No capping is necessary.
Click on the Z box under Direction. This is the axis we lathe on.
Use Center under Align and you want the output to be Mesh.


This completes the Lathe operation so collapse the stack and you now have a rounded end to attach to the main car body.

But you say that the end of the car I am modeling is not vertically cylindrical but more tapered like a bullet.
Well, that is no problem.
Go to Point sub-object mode and down under the Soft Selection section turn it on.
Check off and set the Edge Distance to 1 less the the number of segments on one side of the end.
Remember, I had the number of segments set to 12 for my lathe operation, so I set the Edge Distance to 5.
I also set the Falloff to 5.0' ... think of it as an invisible envelope and the farther a point is from the ones I select,
then the less what I do to my selection affects those far away.
I select ONLY the points that are on the flat edge of the object.


Next I RIGHT click on the Select and Move icon on the Icon Bar to open the Move Transform Type-In dialog box.
I am working in the Top viewport. and I want to move the points that I have selected 5.0' forward from where they are so
I type in -5.0' ( negative being down or to the left depending on the viewport you are working in ) in the Y box under Offset Screen. If I were working in the Right viewport, I would type in the -5.0' in the X box.
Don't forget to tap your enter key to make the change take effect.

Now I have a tapered rounded end.
BTW, I just noticed that the end is facing the wrong way so I applied a Mirror modifier to flip it around the correct way.
Now I can attach and weld the end to the rest of the car body.
If you have a good side elevation drawing you can work against, now is the time to tweak the mesh to fit.
Here is how the carbody looks in the sim.