“Babies should be placed flat for sleep whenever possible, and keeping babies in car seats or other devices for long periods is potentially unsafe for babies in the first few weeks after birth. […] Our advice to parents generally is that babies should not be left in such positions for more than is absolutely necessary for at least the first 6 months.
For families on trains, it is very important that they have the ability to place their young infant in a safe horizontal position for sleep on longer journeys. […] This should certainly include all journeys lasting more than 30 minutes for very young infants (less than a month) and probably more than an hour for older infants up to 6 months of age.”
Professor Peter Fleming CBE
Professor of Infant Health and Developmental Physiology
University of Bristol.