HO scale track is made in two (2) different "codes".

Code 83 is  the most realistic scale for HO track - it is 0.083" high.  It has a shorter height profile than code 100. Code 100 is 0.100" high.

You can tell the difference between them by the color of the ties. Code 83 has brown ties.  Code 100 has black ties.  You can mix the 83 and 100 track by using a tansition track shim.