Police are appealing for witnesses following the collision between a train and a pack of dogs in Llangollen.
Families had enjoyed a day out on the 'Santa Special' from Llangollen Station to Carrog on a decorated steam train.
A spokesman for Llangollen Railway described Sunday's incident as "highly regrettable".
The trip saw children meeting Santa when they got off at Carrog station.
Charlene Jones had been on the train, which left the station at 1.35pm, with her husband Martin and two young children Emily and Seren, along with some friends, ahead of her 35th birthday tomorrow.
North Wales Polices said it was looking for any witnesses to contact them on 101.