A variation of this would be the following formula:

=IF(LEFT(B2,2)="TR:", RIGHT(B2,6),A1+0.1)

assuming that it starts in cell A2. This will get a new transaction
number when it changes, and will then add 0.1 onto it for each leg of
travel - you can always make this 0.01 is you expect more than 10 legs.
Copy this down, fix the values with Edit | Paste Special, then sort the
data set using this column.

Hope this helps.

Pete