Is it obligatory to pay the fines resulting from not having a metro ticket, knowing that the ticket is repeatedly forgotten and payment is waived each time?

If you bought the ticket and lost it upon arriving at the station, you are not obligated to pay the fine, because the purpose of the fine is to discipline those who try to ride the train without paying the fare, and you have already paid the ticket price. Furthermore, the person responsible for the fine authorized you not to pay it and let you go. This is supported by the hadith (definition) of Salama ibn al-Akwa' in the Two Sahihs, that the Prophet, peace and blessings be upon him, ordered the breaking of pots in which the meat of domestic donkeys was cooked, and then, after their inquiry, permitted them to be washed.