Can I bring a bear spray from the US to Canada?

tl;dr: if it's marked as bear spray, go ahead.

The Canada Border Services Agency lists these requirements for the bear spray to be allowed :

Exception: Aerosol or similar dispensers that contain substances capable of repelling or subduing animals (e.g. dog or bear repellent) are not considered prohibited weapons if:

(i) the label of the container indicates specifically that it is for use against animals only; and

(ii) the provisions of the Pest Control Products Act are met.

(iii) In order for a device or dispenser to be considered to meet the above criteria, it must be apparent that the device was designed exclusively for the control or use against animals. The manufacturer’s label and specifications will be deemed as the design of the product. The misuse of these or any other similar device may nonetheless be punishable under other laws.

(iv) Please note that there is no minimum quantity or capacity requirements (in terms of grams or milliliters) in making the determination with respect to animal repellents.

While current Amtrak regulations do not explicitly prohibit bear spray, their prohibited items page lists the following categories of items as prohibited from both carry-on and checked luggage (bolding original):

• Canisters, tanks or other devices containing propellants

• Corrosive or dangerous chemicals or materials, including but not limited to liquid bleach, tear gas, electronic control devices (stun guns, TASER guns), radioactive and harmful bacteriological materials

• Martial-arts and self-defense items, including but not limited to billy clubs, nightsticks, and nunchuks

One could argue that bear spray does not fall exactly into any of these three categories. However, at the top of the page, there is a note that (bolding mine)

The following prohibited items list is not an exhaustive list. Any item that is similar to the prohibited items below, even if not specifically mentioned, is also prohibited. Amtrak personnel may determine if an item not mentioned in this list is prohibited.

So basically, it's up to the Amtrak personnel. If an Amtrak employee learns that you're carrying bear spray, and decides that it falls into one of the above categories (and one could make an argument for each one of them, especially if one wasn't 100% familiar with bear spray), then you might be required to remove it from the train.

That said, what the regulations say and what actually happens can be two different things, as you seem to have found. I'd be curious to know if you actually asked any Amtrak employees about the bear spray, or whether it just flew under their radar.

I would have expected Amtrak to have added bear spray and pepper spray to their list of restricted items in the last 6 years, as in that time and maybe longer there have been internet questions about it. They did not, so it is very likely that they do not worry much about it.
There is a blanket 'items like those on the list' but in my view those are not that closely related.

I would still pack the spray in such a way that it can not be opened/pressed on to use by accident. A ziplock bag just big enough, well closed and see through, and around that a bigger bag with soft items so that it does not have much chance to be damaged by rough handling.

It is taking a risk, as an Amtrak staff member can ask you about what you have in your bags and decide that it is against the rules.

I do not know about Amtrak, but last summer I took the Greyhound from Vancouver (Pacific Central Station, the same as Amtrak and Via Rail trains) to Banff, in the Canadian Rockies.

Before boarding the bus, there are enforced security checks of all the bags. The search is not very thorough but bear spray is clearly a targeted forbidden item. I have seen the security agent confiscate one and he even added he had confiscated a lot (probably many hikers buying some in Vancouver before heading to the mountains).

Greyhound specifically forbids pepper spray, so I suppose like @Willeke said if Amtrak does not forbid it, then it is probably OK. However, if you intend to cross the border to Vancouver and continue to the Rockies, you probably do not need to bother, you will not be able to take it on the Greyhound buses.