Joseph's Wagons: How does Rashi know what he knows?

The verse says:

וַיַּרְא אֶת-הָעֲגָלוֹת, אֲשֶׁר-שָׁלַח יוֹסֵף לָשֵׂאת אֹתוֹ; וַתְּחִי,רוּחַ יַעֲקֹב אֲבִיהֶם
And [Jacob] saw the wagons Yosef had sent to carry him and the spirit of Jacob their father revived.

What was it about the wagons that improved Jacob's mood, and convinced him that his son was alive?



On the spot Rashi (rather famously) cites the midrash that the wagons were proof that Yosef was alive, because the last thing Yosef and his father had studied together was the case of the Egla Arufa and wagons (Agalot), though unrelated to the case, pun on Egel.

He (Joseph) gave them a sign, viz., in what topic he was engaged when he (Joseph) separated from him (Jacob). [That was] the section dealing with the heifer that was to be beheaded (עֶגְלָה עִרוּפָה) (Deut. 21), and this is what [Scripture] says, “and he saw the wagons that Joseph had sent,” and it (Scripture) does not say, “that Pharaoh had sent.” [from Gen. Rabbah 94:3] 

But how does Rashi know that Joseph and Jacob studied this particular topic on the day they departed? Where's the textual clue? (Or to state it like a YU guy, "What suggested to the ancient interpreters that father and son had been studying this subject?")

According to Kli Yakar, the point of the wagons was not to remind Jacob specifically of Egla Arufa, but of the related topic of levaya, or escort. (Eglas Erufa is an offering made by the elders of a town when a corpse is discovered outside the borders of the municipality. The implication is the Elders and residents are partially responsible because they permitted the victim to leave the town unaccompanied, leaving him vulnerable to highway men.)

There's good textual evidence that providing escort to people was a well established custom in Jacob's family. When the angles took their leave from Abraham, the elderly patriarch accompanied them for a bit (And the men rose up from thence, and looked toward Sodom: and Abraham went with them to bring them on the way. Gen 18:16) and, indeed, Jacob escorted Joseph udring the first part of his fateful journey toward Shchem. The verse states

So he said to him, “Go and see if all is well with your brothers and with the flocks, and bring word back to me.” Then he sent him off from the Valley of Hebron.

At the time, Jacob lived in Hebron, which is situated in the hills. If Joseph was dispatched from the valley, the verse can be understood to mean that the father escorted his son into the valley. This, likely, is what Rashi means when he says that Jacob and Joseph were engaged in the topic of Egla Erufa when they separated. Egla Erufa wasn't the last thing they studied together, but a subject closely related to the last thing they did together (Though Kli Yakar does suggest that the topic of Egla Erufa was discussed as Jacob provided his son with escort. Perhaps Jacob explained to his son why escort is important. )

Years later, Joseph demonstrated that he remembers the lesson. When the brothers take their leave from him, to begin their trip back to Canaan, the verse says:

And he sent his brethren away, and they departed [= Vayelechu = "and they walked"]. And he said to them, Do not quarrel on the way.

Seemingly, Joseph sent his brothers off, then walked with them for a bit, after which  he gave them as last warning. In Hebrew, the word Vayelechu is slightly ambiguous, but with no violence at all to the text can be construed to mean that Joseph accompanied his brothers, that is, provided them with escort.  Kli Yakar adds that during this escort, Joseph told his brothers that he and his father had done something similar during the last moments they spent together. As Kli Yakar notes, it wasn't merely the wagons that revived Jacob's spirits, as the verse says, in full:

But when they told him everything Joseph had said to them, and when he saw the carts Joseph had sent to carry him back, the spirit of their father Jacob revived.

"Everything  Joseph said to them", Kli Yakar suggests, includes the memory of the final escort Joseph shared as he escorted his brothers out. The wagons, therefore, were not merely proof that Joseph was still alive, but that he kept up the family customs. It was this additional realization that restored his father's spirit.