If you have trouble getting downloaded Brio tracks to fit well with Ikea Lillabo tracks, try using this cutter in your slicer to make it fit better.
Really, you only need to trim a tiny bit off to make them fit, so I decided to make this model to add as a negative part so you can trim the keyhole side.
I haven't included any actual photos, as the result isn't that different for there to be any point.
if you want to modify it, find it on Tinkercad here: https://www.tinkercad.com/things/dnoYKHn2cb2-lillabo-compatibility-cutter
Step 1: Right click on the model you wish to modify, and select Add Negative Part (most if not all slicers should have this, or similar options.
Step 2: Line up the model with the keyhole. You want to move it as far back as possible, so it's trimming a little bit off the keyhole, and a mm or so off the end of the track.
Step 3: Line up the model with the bottom of the plate. my testing seems to place the model slightly higher
Step 4: Slice your model
The author marked this model as their own original creation.