Currently Spanish national railway company Renfe does not permit people to travel with pets weighing more than 10kg (22 pounds), which means no dogs larger than the average beagle. The only exception is guide dogs for the blind which can ride for free whatever their size.
A petition on Change.org launched less than a week ago has already gathered more than 10,000 signatures calling for the size restrictions on pets to be lifted.
Broadcast journalist Sandra Barneda started the online petition and has been promoting the hashtag #perrosaltren - dogs on trains - to gain support for the reform.
"This summer, many families in Spain will not be able to enjoy their vacations with their best friends: their dogs," the petition states.
"We, as fans and friends of dogs, have an obligation to to give them a voice. And being able to travel with them on vacation is something that should be accessible for all."
The petition proposes reserving certain spaces or even entire train coaches for people travelling with their dogs.
Other countries do not have size restrictions for pets. According to National Rail Enquiries, passengers traveling by train in the UK "may take with them, free of charge... dogs, cats and other small animals (maximum two per passenger) provided they do not endanger or inconvenience passengers or staff."
But Amtrak's guidelines for the United States explain that though service animals are allowed, "Amtrak does not permit pets on trains or Thruway services," with the exception of select Illinois trains.
"A company like Renfe, which has already demonstrated the sensibility to pay attention to the people and offer solutions, has the opportunity to continue being a good model," the petition states.
The plan to give all dogs the right to travel would perhaps also fall in line with the ideology of a Spanish town that recently became the first to grant dogs and cats equal rights among their human counterparts, recognizing them as "non-human residents".