A word of caution.
It's often reported a 'ticket out' is acceptable, whether by air, train, or road.
It's the airline that insist on an onward ticket when entering VE, as if refused entry, it's they who are responsible to return you to your point of departure as well as being fined.
A friend, whose previous extension had lapsed, planned to enter VE > Non O > 1 year extension again.
Aware of the requirement for an outbound ticket, booked an outbound flight to Phnom Phen within the 30 days.
His leg from the UK to Thailand was with Emirates, arriving at Suvarnabhumi.
Ar check-in, he was asked if he had an outbound ticket, which he had.
However, they noted the outbound flight was from Don Muang to Phnom Phen.
They asked, if refused entry at SV, how he intended to travel to DM to catch his outbound flight.
He was denied boarding until he rebooked an outbound ticket departing SV.
The same would apply to train or road travel outbound tickets.
If refused entry, how would you travel to the train or coach station to catch that outbound transport.
Of course, the chances of being refused entry are low, but it then becomes the airline's responsibility if you're unable to use any outbound ticket.