1. Earlier You wrote 20978 has 20 % till Jaipur , which is not factual .
2. We know how % works ,and here 20% denotes to 20/100 or similar ratio. This means 80 out of every 100 seats remain vacant . Which may only be true for Jaipur Ajmer leg of the journey which comes after Jaipur. Hence post Jaipur.
Check “ a prefix, meaning “behind,” “after,” “later,” “subsequent to,” “posterior to,” occurring originally in loanwords from Latin ( postscript ),”