The NEW St Pancras station is a short walk up Pancras Road from the original station. you cannot miss it as there are huge hoardings hanging off the side of the old station.

Once you get onto the station there are barriers but these are normally only manned when trains are boarding. If you ask nicely they normally let you through if you say you want to get the numbers of the stock.

If you want to have a look at the Javelins from the South Eastern platforms you can normally get allowed through. I've never had a problem.

The Thameslink trains including 319's and 377's go from the underground platforms. If you want a variety of spotting I would recommend Kentish Town

ST Pancras has now become a very complex station so I though an update was in order.
Excluding the underground it is on 2 levels and with 4 platform sections.
The exit from the underground and the entrances to the FCC (Thameslink) and Eurostar are on the lower level , at the southern , eastern and northern ends respectively.
The new Kent coast service is in the North-Eastern corner up escalators.
On the upper level are 13 platforms numbered West to East.
1 to 4 seem to be the Midland service (accessed from this level), 5 to 10 the Eurostars - totally secure at this level but with the south end of the trains visible. 11 to 13 are the Kent service more or less opposite the Midland ones but the Eurostars are so long that you can't normally see across. The domestic ticket barriers are on this level for these platforms so you can see the buffer stop end for the Javelins without a ticket. If coming from King's Cross you don't need to go into the big concourse area for these , just keep on up the road past the Eurostar entrance and the escalators are immediately in on the left through a glassy entrance.
The FCC platforms are called A and B and you need a ticket to see these.

I have done further exploration and if you need the 12-car 395s , you can see them easily if you go out of the staion on the east side , head north through the traffic lights and there is a spot near the road tunnel where they can be seen . Some Eurostars can too but are a bit low down and far away so bins help. about a quarter-mile.

St Pancras is currently undergoing MAJOR re-building work, to make way for the new Eurostar Tunnel Terminal, therefore only four platforms were in operation on the day that I visited, although I counted at least 13 new platforms under construction.
A Midland Mainline station, so plenty of HST's 221's and 222's when I visited albeit very briefly.
Plenty of food outlets and of course London is just outside the doors!
I found the staff very freindly and helpful, and had no problem accessing the platforms the day I visited.
The one thing that amazed me was how light and airy the whole building was, ideal for non-flash photography which seems to be the main bugbear in most stations these days!
A station for the 21st Century, an ideal spot in times to come!
My rating 8½/10.

Good place to see passenger trains

Main access is the local buses and London underground.

There is a snack shop at the station.

Please note that no matter what you read to the contrary, you will be asked to move off the platforms by security people. This has happened before, and has today (25/11/2003).
BREN.