Paddington to Windsor Castle
Paddington to Windsor is 30 minutes by fast train with one easy change at Slough.
Waterloo to Windsor is 53 minutes with no change.
There is no advantage in buying tickets in advance.
For very detailed information, read on.
The fastest way to get to Windsor by rail is from London Paddington to Windsor Central Station. This involves one easy change at Slough; just walk to platform one and get on the Windsor train. Windsor Central Station is located just beneath the Castle walls. The entire journey can take under 30 minutes. This route is serviced by First Great Western Rail, with roughly three journeys per hour.
Another option is to take the rail from London Waterloo to Windsor's second rail station, Windsor and Eton Riverside station. This takes longer, typically 53 minutes, but is direct to Windsor. Trains run half-hourly Monday-Saturday, less regularly on Sundays. The Windsor and Eton Riverside Station is only a five minute walk from the Castle, but this involves a very steep hill. This route is serviced by South West Trains.
As Windsor is outside zones 1-6, a travelcard will not cover your journey there. However, there may be a small savings available as the traveler can purchase an extension ticket to Windsor; this would be the fare from the last zone in the traveler's travelcard to Windsor. You would purchase an extension ticket from either Paddington or Waterloo, and take the train from there. At this writing, the savings is £3.
There is no advantage to advance booking, but you can choose to buy your tickets online at a train operator's website and collect at the station. You will still need to queue for the machine (which could sell you the ticket anyway) to collect them.
2015 fares (only return fares quoted, as they are cheaper than two singles) are:
Paddington - Windsor & Eton Central
£13.90 Anytime Day Return (leaving Paddington weekdays before 09:20).
£10.40 Off-peak Day Return (valid any time weekends, and on the 09:21 (arriving Windsor 09:59) or later weekdays).
Waterloo - Windsor & Eton Riverside
£18.30 Anytime Day Return
£11.70 Off-peak day return (valid leaving Waterloo on the 09:28 or later weekdays, any time weekends)
Child fares are always half the adult. Children age five and over require a ticket on the train.
If travelling with a group of three or more, then Groupsave will reduce costs. With Groupsave you pay for two adult tickets and get one or two free (according to how many are travelling). Groupsave is only available in conjunction with Off-Peak fares.
Travelcard extension fares are as follows:
From Paddington to Windsor & Eton Central:
From Zone 2: £9.70 off-peak day return (valid 09:30 onwards);
From Waterloo to Windsor & Eton Riverside:
From Zone 2: £10.50 off-peak day return (valid 09:30 onwards);
To buy an extension fare you need to go to the ticket office and request it - they are not sold at machines.
Southwest trains Rail + Castle tickets
If you buy a ticket to Windsor & Eton Riverside or Windsor & Eton Central from Waterloo or Paddington, you can add on a reduced price admission to Windsor Castle from the station ticket office, saving £3.35 per adult ticket.
However, if you are travelling as a family..2 adults and up to 3 children under 17 years of age, priced at £49.70 and sold at the Castle, the reduced price admission by Southwest trains may not be to your advantage.