Need help downsizing an OBJ file I am a complete novice to 3D. I am trying to downsize a file of a train engine that is 400+MB so that I can import it into Adobe Dimension for a layout. Currently the file is too big to be used practically.

I've tried Decimate but I know I'm doing something wrong because I can only get one section to downsize, not the entire train. I've tried eliminating the hidden polys, but no luck there either.

I get the files from a third party (not sure what software is used originally), then an engineer converts it from either STP or IGS to OBJ. If anyone knows a better way to achieve a smaller file, it would be a great help. Thanks!
One thing I could recommend is joining all of the meshes (selecting them all) and then pressing control j (join meshes). This will work if you have a lot of different objects. My guess is that you only applied the decimate modifier to one specific object and that you are only decimating that part. Could you share a screenshot or two so I could see better what the problem might be?