Yes, you are right. Site won't allow you to use the same number for two separate trains. In order to solve this dilemma, first of all you have renumber the previous old train to slightly different number by using X, XX etc in front or at the back of the original number. For example, from 12345 to 12345X or 12345XX or X12345 etc.
As soon as the old cancelled train gets renumbered, the original number gets clearance to create the new page for the new upcoming train.
...
more...
Otherwise the old train details get washed away by the new train details, and that what is called as page merging.