Thank you both for these clues.  I also recollect mention of palace cars 
some time ago on Twain-L but I don't have the references.  I have found 
myself immersed in railroads while tracking Twain's travels - more than 
I had ever intended.  And, for the past week I've been in a 
semi-delirium from a break through case of covid  subjecting me to odd 
trajectories prompted by skimming through Day By Day for times and 
places.  Of course it's these colorful little tidbits that add so much 
to delving into Twainiana.  I suspect the palace cars were manufactured 
by the Pullman Company.

On 8/21/21 6:03 PM, Matt Seybold wrote:
> I don’t have the references in front of me, but the Clemenses used the Langdon family’s private cars regularly in the 1900s (and probably before as well). I believe there is a letter in which Twain describes one in some detail as part of planning to transport Livy back to Elmira while she was convalescing.
>
>   - MS
>
>> On Aug 21, 2021, at 8:48 PM, Barbara Schmidt <[log in to unmask]> wrote:
>>
>> The reference you may be thinking of is mentioned in a letter to John
>> McComb 6(?) Feb. 1870.  The original letter is apparently lost.  The
>> transcription from newspaper publication is online at:
>> https://www.marktwainproject.org/xtf/view?docId=letters/UCCL00420.xml;query=Palace%20car;searchAll=yes;sectionType1=text;sectionType2=explanatorynotes;sectionType3=editorialmatter;sectionType4=textualapparatus;sectionType5=;style=letter;brand=mtp#1
>>
>> Barb
>>
>>> On Saturday, August 21, 2021, Scott Holmes <[log in to unmask]> wrote:
>>>
>>> I've gotten hints that Mark Twain had available to him, at least briefly,
>>> a luxury railroad car, probably made available to him by his
>>> father-in-law.  I would appreciate any leads on this as Google seems to
>>> know nothing about it.  I did see the term "hotel" used to describe a car
>>> Twain rode in, possibly  from Elmira to New York, if I'm not mistaken.
>>>