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