This week, an Adelaide rail passenger was spotted with a snake on a suburban train — apparently in breach of transport regulations.

Key points: Guidelines regarding pets on public transport are determined on a state-by-state basis

Guidelines regarding pets on public transport are determined on a state-by-state basis Some states are much stricter than others, with the issue still being debated in Queensland

Some states are much stricter than others, with the issue still being debated in Queensland In Victoria, commuters with dogs must keep them muzzled

The photo posted on a popular social media site showed the man holding what appears to be a python.

Across Australia, there are different regulations about what kinds of pets you can take onboard public transport.

Some require pet owners to seek permission, and ensure their animals do not pose a threat, while others have imposed blanket bans on anything other than assistance dogs.

In the case of the snake, the incident was not flagged with police.

An assistance dog boards a train with a passenger in New South Wales. ( Transport NSW )

The Department of Planning, Transport and Infrastructure (DPTI) said the man had not sought approval to bring his serpent on the train.

"We are not aware of this person carrying a snake onboard a train," it said in a statement.

Knowing which pets are allowed onto public transport, however, is not always an easy task because of differences between state laws.

What do the biggest states do?

Some states are more flexible than others when it comes to letting animals, especially pet dogs, travel on public transport.

There is little, if any, mention of snakes — although that might be because reptile ownership is subject to separate state regulations.

In New South Wales, pets are allowed to travel on buses, ferries, in taxis and on trams provided they are stored in boxes, baskets or other containers — and permission is given by staff or the driver.

"Animals that are not clean, appear vicious or likely to annoy, threaten or inconvenience other customers will not be allowed," Transport for NSW states on its website.

"Animals are not allowed on seats and must not obstruct passageways.

"The driver or crew may refuse to allow the animal if the service is reaching capacity."

Sniffer dogs search for drugs on a train. ( ABC News )

But people requiring assistance animals do not have to seek prior approval.

"Assistance animals with valid accreditation can travel free on all metro, NSW train, bus, ferry, light rail, taxi, private bus and private ferry services," regulations state.

That includes guide dogs, but also dogs for people with hearing impairments and those which help people with mental health conditions.

Police dogs and sniffer dogs are also allowed onto public transport.

Victorian regulations are similar, allowing small animals in "suitable" containers onto metropolitan trains, trams and buses — but bigger dogs do not necessarily have to be caged.

"Dogs can travel with you on trains, as long as they're on a lead and wearing a muzzle," Public Transport Victoria's website states.

Commuters with dogs are advised to avoid peak hours and to "clean up any mess your dog makes".

Debate over dogs on CityCats

Not all states have resolved the issue of whether animals should be allowed on public transport.

Earlier this year, the RSPCA called for dogs — but not other pets — to be allowed on all forms of public transport throughout Queensland, including Brisbane's famed CityCat ferries.

The RSPCA in Queensland is pushing for dogs to be allowed on public transport. ( 612 ABC Brisbane: Jessica Hinchliffe )

It prompted community debate, with some eager to take their pets for rides and others deeply opposed.

"We understand that not everyone wants to be near them [dogs] and some people are allergic or don't understand dogs," RSPCA spokeswoman Alex Hyndman-Hill told the ABC in February.

"We're not saying you're going to get a rottweiler next to you on the ferry."

A petition to allow dogs on board received more than 2,000 signatures.

In Western Australia, transport authorities require their owners to "carry a valid photo ID card of your dogs".

"Your dog must have a suitably marked dog coat or harness that identifies it," Transperth states.

In the NT, dogs for people with disabilities are allowed but "all other animals are prohibited", the Northern Territory Government's website states — there are no provisions for crocodiles.

Tasmania has similar regulations, with bus operator Metro Tasmania informing passengers they must not "bring an animal on to a vehicle unless it is an approved guide or assistance animal".

The debate about pets on public transport is one that has also been had in some of the world's biggest cities.

In New York, the city's Metropolitan Transportation Authority (MTA) allows commuters to carry animals on the subway, with a couple of conditions.

"No person may bring any animal on or into any conveyance or facility unless enclosed in a container and carried in a manner which would not annoy other passengers," the MTA's website states.

What the bureaucrats responsible for drafting that rule may not have foreseen is that the word "container" has become widely interpreted as meaning "bag".

"It is impossible to quantify how many dogs are riding the rails," a reporter for The New York Times wrote in 2016.

It is now a fairly routine sight to see pooches on the city's subway network stuffed inside sports bags and even designer handbags.

Dogs in bags are a common sight on New York's subway network. ( Flickr: Charley Lhasa )

What about the snake?

According to Adelaide Metro guidelines, animals other than those assisting people with disabilities are not allowed on — unless their owners have permission.

The sighting of the python caused a flurry of responses on popular Instagram page 'Shit Adelaide'.

"Could be the star of the new movie, Snake on a Train," one person wrote.

"Great way to make sure no one sits next to you though!" another said.

Other social media users were similarly startled, with one stating that this was "just one of the many, many reasons I don't use public transport in Adelaide".

"I take this train twice a week. I guess I will jump at the slightest move now," another said.

DPTI warned that breaches of its rules regarding animals could lead to fines of up to $750.