It depends on the context, but usually "on the train".

Here's a way to remember traveling prepositions: 1. If you can stand or walk on it, we use the preposition "on" (e.g., a train, plane, ship, subway) 2. If you have to sit in it, we use "in" (e.g., a car, a truck, a rowboat) 3. If you actually sit or stand on top of it, we use "on" (e.g., a motorcycle, a bicycle, a skateboard) 4. We use "by" to indicate the method of transport (by car, by plane, by sea, by air, by bike, etc.).

both of them 1) example of a poem The Boy in the Train by Mary Campbell Smith Whit wey does the engine say 'Toot-toot'? Is it feart to gang in the tunnel? Whit wey is the furnace no pit oot When the rain gangs doon the funnel? What'll I hae for my tea the nicht? A herrin', or maybe a haddie? Has Gran'ma gotten electric licht? Is the next stop Kirkcaddy? There's a hoodie-craw on yon turnip-raw! An' seagulls! - sax or seeven. I'll no fa' oot o' the windae, Maw, Its sneckit, as sure as I'm leevin'. We're into the tunnel! we're a' in the dark! But dinna be frichtit, Daddy, We'll sune be comin' to Beveridge Park, And the next stop's Kirkcaddy! Is yon the mune I see in the sky? It's awfu' wee an' curly, See! there's a coo and a cauf ootbye, An' a lassie pu'in' a hurly! He's chackit the tickets and gien them back, Sae gie me my ain yin, Daddy. Lift doon the bag frae the luggage rack, For the next stop's Kirkcaddy! There's a gey wheen boats at the harbour mou', And eh! dae ya see the cruisers? The cinnamon drop I was sookin' the noo Has tummelt an' stuck tae ma troosers. . . I'll sune be ringin' ma Gran'ma's bell, She'll cry, 'Come ben, my laddie', For I ken mysel' by the queer-like smell That the next stop's Kirkcaddy! 2) example; http://en.wikipedia.org/wiki/On_the_Train_(poem) On The Train is a poem by Gillian Clarke. Its chief subject matter is the Paddington rail crash and its aftermath.

on on a train/ on a bus

'I'm ON the (a?) train' is correct.