Commuters won't be given a free travel day after Melbourne's rail network meltdown, even though a previous major incident led to such compensation.

An estimated 175,000 people were left stranded or delayed when the city's train system was brought to a standstill on Thursday afternoon.

It's understood an investigation into the cause of the failure remains focused on whether a back-up computer system may have triggered the glitch. Metro has sent its data overseas for further analysis.

Passengers who touched on between 3pm and 7pm will be reimbursed on their myki account, but anyone who didn't touch on will have to fill out a form to claim compensation to be decided on a case-by-case basis.