Going further than I had the time to yesterday, you might also want to know the following.
I'm looking at the 1851 census, and there are some other children who are listed there and whose births are registered to Alexander Trail and JEAN (not Jane) Downie. I'd be confident the name is an error, and these are also the children of Alexander and Jane. Including the already listed children (William, John & Jean) for completeness:
Margaret TRAIL born 7 Jul 1833, christened 15 Jul 1833, Pitsligo.
William TRAIL born 1 Jul 1835, christened 8 Jul 1835, Pitligo.
John Downie TRAIL born 14 Aug 1838, christened 21 Aug 1838, Pitligo.
Jean TRAIL born 15 Aug 1840, christened 22 Aug 1840, Pitligo.
Ann TRAIL born 6 Aug 1846, Pitsligo.
Jessie TRAIL born 21 Feb 1849, Pitsligo.
Alexander TRAIL born 22 Nov 1853, Pitsligo.
Peter Sim TRAIL born 28 Aug 1855, Pitsligo.
I'm estimating Alexander to have been born about 1803 to 1813 (using the unscientific method of removing 20 to 30 years from the birth of his known first born). I'll also assume he was also born in Pitligo. There's an Alexander TRAIL born to a William TRAIL and Jane (possibly JEAN) RITCHIE in 1808. Strong candidate given his children's names. There's also an Alexander born to Alexander TRAIL and Christian DUTHIE in 1817. This would have meant Margaret would have been born when her father was only 16, and the name Christian isn't given to any of Alexander's children.
There's a Jane Downie born 2 Dec 1810 to John DOWNIE and Ann SIM. Taking this Jane, this would suggest Alexander we want is 1808:
Margaret - unaccounted for (but see Alexander's eldest sister born 1798)
William - named after paternal grandfather
John - named after maternal grandfather
Jean - named after paternal grandmother
Ann - named after maternal grandmother
Peter's middle name Sim - named after maternal grandmothers m.s.
So now we can add:
William TRAIL and Jane/Jean RITCHIE married 23 Nov 1797, Pitsligo.
Margaret TRAIL born 3 Nov 1798, Pitsligo.
Alexander TRAIL born 25 Aug 1800, Pitsligo.
Christian TRAIL born 21 Nov 1801, Pitsligo.
Jean TRAIL born 5 Jan 1803, Pitsligo.
Ann TRAIL born 29 Jun 1805, Pitsligo.
Alexander TRAIL born 4 Jun 1808, Pitsligo.*
John TRAIL born 23 Jun 1811, Pitsligo.
Barbra [sic] TRAIL born 2 Jul 1815, Pitsligo.
*This is the second Alexander to the family, which means Alexander born 1800 has died by the time this Alexander was born. Obviously this Alexander is your grandfather.
Finding William's birth is not so easy, and should be considered a guess. The BMDs should give more clues. However, using the 20-30 year guess, and looking in Pitsligo only, a John TRAIL and Grizel/Grizzel RITCHY/RITCHIE have the following children:
William TRAIL born 20 Mar 1771, Pitsligo
Alexander TRAIL born 23 May 1775, Pitsligo.
There's no indication in the names of William and Jane/Jean's children that this is the correct birth. But then, William apparently doesn't name any children after himself.
Similiary, take the following with a pinch of salt:
John TRAIL born 13 Jun 1741 to John TRAIL and Janet RITCHIE, Pitsligo.
That's as far as the easy stuff goes on the IGI. You should check all my dates add up, and grab the copies of the originals from Scotland's People. The details on the birth, marriage and death certificates will not only help to confirm the ancestry (using names, locations and occupations), but might tell you more about your ancestors. You should also look for censuses between 1841 and 1911 (1911 available next week).
You ask about George and Wilson Summers. To find out what happened to them, you'll need to look on Scotland's People. You know approximately when and where they were born, so you should have no problem geting their birth and death records. It's probable that you wont be able to see their death records online, so you'll need to find when they died (which given their year of birth should be fairly easy if they stayed in the Pitsligo area) and from there order an extract. If ordered from Scotland's People, the cost is currently £10, going up to £12 next week.
Hope that all helps. As I wrote, do check all the information I've given you. I've not done as much genealogy as I'd like to as my own Scottish ancestry is proving more elusive, but I hope I've got you started.
Tim