Thank you, Barbara.  This does look like the earliest use of a special 
car - four years later than indicated in Day By Day. Fears got it from 
Willis (the attribution is in his text) and apparently Willis found this 
letter but apparently forgot the date it was written.  Near the bottom 
of the page Miki sent me is a note that Susie was four years old at this 
time - 1876 not 1880.

On 8/23/21 3:16 PM, Barbara Schmidt wrote:
> Scott,
>
> Rest Willis does quote the passage about Livy always going by special car
> hereafter and sources it to a June 17, 1880 letter to Lilly Warner. It is
> online here —
> https://www.marktwainproject.org/xtf/view?docId=letters/UCCL01814.xml;query=Warner;searchAll=;sectionType1=;sectionType2=;sectionType3=;sectionType4=;sectionType5=;style=letter;brand=mtp#1