I've changed the last line of verse 2, added some punctuation and a three line final verse.

I can hear a sound from childhood:

A mixture of machine and animal

And recognise a steam train

Getting closer, closer, closer…

I turn the engine off

To hear more clearly the chuff chuff chuff

And clank clank clank of rods.

It’s here! A great black beast vomiting

Choking clouds of smoke and cinder,

A dragon monster breathing fumes

And in between, the glints of brass and steel.

My eyes begin to smart, I cough,

Turn off nostalgic memories

And wind the window up.

I can hear a sound from childhood

A mixture of machine and animal

and recognise a steam train

Getting closer, closer, closer…

Removed the ‘I’ from line 3, replaced with ‘and’.

At the level crossing, paused, eager

I turn the engine off

To hear more clearly the chuff chuff chuff

And clank clank clank of rods

Changed the connection with clank clank line to link it more definitely with the previous line and the positive image.

Before the contrast:

It’s here! A great black beast vomiting

Choking clouds of smoke and cinder

A dragon monster breathing fumes

with glints of brass and steel

I’ve altered quite a bit here. ‘Vomiting’ is a fierce choice but I think it works.

That’s been a useful way of comparing versions.  I’ve made these choices:

I can hear a sound from childhood

A mixture of machine and animal

I recognise a steam train

Getting closer, closer, closer…

At the level crossing, paused, eager

I turn the engine off

To hear more clearly the chuff chuff chuff

With a clank clank clank of rods

Choking clouds and cannon volleys

A black beast with glints of brass and steel

It really is a dragon monster breathing fumes

Now to polish it further…

An experiment.  Rather than edit and lose previous versions, I've tried putting the original in oine column and possible changes in the other. Hardly rocket science but maybe quite helpful...

Some thoughts and notes following last week's start:

I turn off the engine all the better to hear

The magic chuff chuff chuff

And the whistle, redolent of the recent(?) past

And then it’s here, with a clank of connecting rods

And steam and smoke and cinders.

It really is a monster, a dragon breathing choking fumes

A black beast with glints of brass and steel

Visible through clouds / volleys of

I realise, however appealing now in ones and twos

In general, these were dirty, polluting, unhealthy things

Ivor the Engine Thomas and his friends were

I’m quick to wind the windows up and gaze from a distance.

I can hear a sound from childhood

A mixture of machine and animal

Of course I know immediately

It's a steam train, getting closer

Paused by the level crossing, we wait eagerly

Already in this swift outpouring of words, I have to make judgements I really don't want to slow down to make. Immediately or straight away? Steam engine or train - steam engine could be stationary - too much knowledge. Almost put 'wait in eager anticipation' - surely a cliche and a tautology?

Away for a few days, can cogitate on this and more.