Any suggestions for top 3 railways "things to do" in Sydney

["Peter Mason" <Peter_Mason@nag.national.com.au>]

My son (8) and I (we are from Melbourne) will be in Sydney for a three days
toward the end of the month. We will be staying in Darling Harbour, but
would like to do some railway oriented sightseeing. We arrive Sunday morning
and depart via XPT Tuesday evening.

This is probably asking for trouble, but would any of you care to suggest
the "Top 3 railway things to do in Sydney and environs"?

Please try to keep the costs reasonable, and we will be using public
transport to get around. As we are returning to Melbourne on Tuesday 25 Jan
on the overnight XPT, any suggestions to improve the XPT trip also welcome.

Thanks in advance,
Peter