Rabbi Weinreb discusses the question and asks what is is about egla arufa that causes Rashi to make the connection to the wagons. Is it only a word play and what would be the message that Yosef is trying to convey?
Rabbi Weinreb explains that the message that Yoseph conveyed with these wagons was that he had learned the lesson of personal responsibility for everything that he was involved in. The meforshim explain that Yaakov saw that Yosef had not just sent the wagons of Par'o to get them (from the markings on the wagons) but had ensured that the symbold of avodas zarah had been removed from them. Had he just ordered wagons from Par'o's stables, the normal avodas zarah marking would have been left on. It was the complete setup of the wagons that showed Yaakov that Yosef had been personally involved with sending them.
Rashi wonders what it was about the wagons, the agalot, that convinced
Jacob and revived his spirit. Rashi tells us that these wagons were a
sign sent by Joseph to Jacob, recalling the subject of their learned
conversation when they first parted ways so long ago.
That subject is the ritual of the “calf [Hebrew egla] with a broken
neck,” the details of which are described in the first several verses
of Deuteronomy 21. Joseph was apparently confident that Jacob would
see the connection between the word for wagons, agalot, and the word
for calf, egla.
The reader of Rashi’s words cannot help but ask with astonishment: Is
this some game, some bizarre wordplay? Agala calls to mind egla? What
connection can there be between the ritual of the calf and Jacob’s
parting words of instruction to Joseph before sending him off on his
mission to his brothers, never to see him again until this moment?
...
As the Kli Yakar explains, if the elders of the city are not
hospitable to the wayfarers who frequent the city, the criminals who
populate the environs of the city will assume that this wayfarer is of
no import, and they will therefore take liberties with him, even to
the point of shedding his blood. Were these villains to observe that
the wayfarer was significant enough to the elders of the city to be
treated graciously, they would have refrained from harming him.
This is the nature of responsibility. The elders are not suspected of
actual murder. But if they treat their guests improperly, they set in
motion a process by which those guests are dehumanized, becoming easy
prey to malicious persons. That is how far the demands of
responsibility extend.
When Jacob sent Joseph on his dangerous mission, continues Kli Yakar,
he escorted Joseph part of the way. By doing so, he was teaching
Joseph the lesson of the “calf with a broken neck,” the lesson of the
importance of escorting the traveler, thus demonstrating the human
value of that traveler. Joseph signaled to his father that he learned
that lesson well and knew the responsibility entailed in dealing with
one’s fellow.
Jacob realized that it was Joseph who personally had a hand in sending
the wagons of Pharaoh, thereby escorting his brothers part of the way
back to Canaan. Jacob took note of those wagons and therefore knew
that Joseph had learned that a minor gesture of considerate behavior
to others may have long-term consequences. He signaled that he had
learned the crucial importance of taking responsibility for all one’s
actions, however insignificant they may appear. And so, “The spirit of
their father Jacob revived.”
Agalot and egla are not just words in a linguistic game. Rather, they
allude to the profound lesson about personal responsibility, which is
the basis of the requirement of the elders to proclaim their innocence
of murder.