On Thursday, Bush's casket will travel back to Houston, where he will be memorialized at St. Martin's Episcopal Church and eulogized by his grandson, Texas Land Commissioner George P. Bush, and former Secretary of State James Baker, a man who has been by Bush's side since the former president's failed 1970 Senate campaign in Texas.
His remains will arrive in Washington on Monday, and he will lie in state at the Capitol through Wednesday.
Barack Obama, who served as President from 2009 to 2017, and Bill Clinton, who took over from Mr Bush in 1993 and served until 2001, are expected to be at the service alongside George W Bush (01-09).
Once Bush lies in state, his casket will be transported by motorcade on Wednesday morning to the National Cathedral, where a state funeral will be held. This website is not intended for users located within the European Economic Area. President Donald Trump and wife Melania Trump are scheduled to attend.
Sully started serving as Bush's companion in June after Bush's wife, Barbara, died in April.
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According to Sully's Instagram account, he also spent a lot of time frolicking on the beach and playing fetch.
His son and 43rd U.S. president George W Bush posted an Instagram photo of Sully lying in front of his old master's casket on Sunday.
The entire campus, except for dining services, will be closed for the day.
Shortly before leaving the service, he married his 19-year-old sweetheart, Barbara Pierce, and forged the longest presidential marriage in USA history.
Historians believe Harry S. Truman's folksy aphorism about friends in Washington and dogs is probably apocryphal - the 33rd United States president didn't actually like dogs - but he would surely have fallen for Sully's charms. Retired Union Pacific General Director Mike Iden said that while touring the locomotive after its unveiling in 2005, Bush paused in the conductor's cab. The elder Bush's body will be buried in a family plot at the George H.W. Bush Presidential Library and Museum in College Station. He was denied a second term by Clinton, who would later become a close friend.