1. First, check that your line is really disrupted
Before doing anything, confirm the disruption: proof only makes sense if the line really is running late. Check your line in 2 seconds (live traffic status + next train), or see every disrupted line, live.
2. The official certificate (RATP / SNCF / Transilien)
Operators issue a delay certificate (“attestation de retard”, sometimes called a “bulletin de retard”) when the disruption is confirmed and goes over a certain threshold:
- Metro / RER (RATP-run sections): request it online on the RATP website, in the “Attestation de retard” section.
- RER / Transilien (SNCF-run sections): the certificate comes from SNCF Transilien, depending on the line and the section concerned.
It's free, dated, and it's the most “official” document to show your employer.
3. Keep a record on the spot
No time to wait for a certificate? Open your line's page (for example Line 13 or the RER A) and take a screenshot: it shows the traffic status, the official Île-de-France Mobilités message (reason, section affected) and the time of the last update. It's quick personal proof, not to be confused with the operator's official certificate.
Bulletin, attestation, justificatif: what's the difference?
In practice, they're closely related French words for the same need: proving a public transport delay. “Attestation” and “bulletin” refer to the operator's official document; “justificatif” is the general word your employer will use.