If you only want the track number use as
format string: $num(%track%,2)_%title%
Edit: Some further explanation:
Using only %track% would actually lead to 01/09 ... if the / would not be an illegal character for filenames. So that is cut out und you get 0109.
The $num() function transforms the string 01/09 into a number which means it stops at the first non-numeric character, the /.
The 2 in $num(%track%,2) sets the padding.