That looks like 3doEd by Dave Noonan. That is a program that can be used to load and display tracks and individual objects. It can also do some simple editing like changing geometry points manually (by entering new coordinates), moving objects around (within limits, because from which points on the track the objects are drawn can not be changed). You can also use it to change the base colours of the polygons, but not the textures themselves. That's about it as far as functionality goes IIRC.
So what CowTownBob mentioned, adding 3do objects to a track with 3doEd is not immediately possible. I have done that in the past by merging my new objects with existing ones that are close to it. I did that by making a small .3do file myself without any geometry in it. It only referenced both the old (but now renamed) .3do object and the one I tied to it. I think I used the 3DO->TXT->3DO tools. Mind you, this was in the days before we had the Sandbox at our disposal. You need flavour 15's in the new 3do to reference the other objects if I remember correctly.
Anyway, when you build a track with Sandbox, you also use a graphical program called OPE.exe that is included. It uses the 3D file. You can add track side objects and things like skid marks on the track. You are placed on the track and can use a joystick to move back and forth to place objects. Their general position is usually enough, because OPE is not a very efficient program and we can use 3doEd to perfect the placement while looking at it from different angles.
Maybe this will help a bit to understand the workflow of the sandbox:
Step 1: SGE.exe -> Uses the SG file - Create the layout of the track by making a centerline and changing the elevation of the track.
Step 2: SFE.exe -> Uses the SG file - Create the actual track by adding surfaces and walls.
Step 3: Rinse and repeat. You can use both SFE.exe and SGE.exe repeatedly to fine tune the layout and surfaces.
Step 4: Build the TRK from the SG using SG2TRK and the 3D file from the TRK file using TRK23D.
Step 5: OPE.exe -> Uses the newly created TRK and 3D files - Add extra visuals like skid marks and track side objects
Step 6: Build the 3DO file from the TRK and 3D file. The game uses only the TRK and 3DO file. The TRK file defines the physics part of the track and is completely seperate from the 3DO file, which defines the visuals. That means both the SG file and the 3D file are only used by the Sandbox to edit the track and are not used by the game itself.
Edit: it has been so long... but I think back in the day I had a way to reload objects for a track into one created new with Sandbox. I think it involved exporting a list from 3doEd and adding them to a 3D file manually... Maybe Wolf_PD can confirm
