Now, everything has changed for man's best friend. It can come and go as it pleases, as long as the rules are followed.
And here are the rules.
The owner has to make sure the dog is muzzled all the time. And it has to be on a leash, no more than 50 cm long. It cannot travel on any random carriage -- only the last carriage of a train allows canine entry. Also, the dog has to be registered with the city authorities.
The timings are significant, too.
Dogs still can't ride the metro at all times. Peak hours are a strict no-no. So, from 7:30 to 9:30 in the morning, and 2 to 4 in the afternoon, and 6 to 8 in the evening, no dogs will be entertained on the metro. However, weekends and holidays would not see any such restrictions.
Predictably, the decision has been controversial. While animal lovers and dog owners have welcomed the move, quite a few Madrileños are hopping mad.
Cifuentes's Facebook post on the topic has generated over 600 comments, with many commenters coming out strongly against the move.
The fear of allergy is uppermost on several minds.
Says a particularly peeved commenter Klenio Giffoni, "The people who are allergic to dogs will have to stop using the subway fearing an asthma attack? Because even though during rush hour the dogs aren't using this service, the hairs of the dogs are left by all parties." (A rough translation from Spanish)
Then there is the fear of feces.
Apocalyptic visions of commuters wading through doggy droppings or slipping on doggy urine seemed to be seared on many a brain.
Asks a commenter named Marisa, "What will happen in a carriage full of people when a dog pees? Will the owner take out a paper towel and dry it?"
Felipe Pascual boldly goes one step further and asks a question that many want to know the answer of but are too polite to ask: "Who will pick up the excrement?"
Excrement is a touchy topic with the dog owners of Madrid. A few years back, the mayor of a Madrid suburb had initiated a unique drive, wherein dog poop was picked up from the streets, packed in huge white boxes, and gifted back to the dog owners who had failed to pick them up originally.
Will similar measures be undertaken in the metro?
Unlikely, as of now.
Emilio Castillo, another commenter on Cifuentes's post, paints a grim picture of the future.
"We'll have to see how many leave without picking up after their dog. If we are just as dirty as in the streets, the subway is going to turn into a s**thole."
However, to each negative response, there has been a slew of positive responses lauding the dog-friendly initiative and defending Cifuentes's move.
As one commenter cheekily put it apropos the allergy debate, "And what about dogs who are allergic to people?"
Indeed, what about dogs who are allergic to people?
[Photo by Dan Kitwood/Getty Images]