How about this?
$Directory = "d:\rootfilepath"
# this is not really the preferred way
Invoke-Command -Computer SERVERNAME -ScriptBlock {& "D:\incomingdatafiles\datafileloader\datafileloader\callDatafileUploader1.ps1 $using:Directory"}
# This is safer (and preferred)
Invoke-Command -Computer SERVERNAME -ScriptBlock {Start-Process -NoNewWindow -FilePath "D:\incomingdatafiles\datafileloader\datafileloader\callDatafileUploader1.ps1" -ArgumentList $using:Directory}