As for the type of wood, in other applications I've had trouble using oak against steel and iron. It causes corrosion. And don't even think of using treated lumber. I think I've also read that redwood will cause corrosion. My assumption is that once you install the wooden blocks they will stay for a long time.

Brasses occasionally come up for sale here in the classifieds. We offered some and as far as I know, there were no takers. I don't know if we still have them.
viewtopic.php?f=2&t=42641&hilit=journal

Steel on steel will fully tear up the machined surface of axle, most likely beyond any ability to be able to repair and reuse the axle ever again.

In the interest of answering the OP's question and situation, I would gravitate away from a hardwood for cutting in the field, but a little heavier than really junky softwood. I don't want to know what happened to a journal when plywood glues on a softwood heated up and liquified.

Here is a hardness chart. My guess would be somewhere between Birch and Cherry for field cutting and fitting, maybe Ash is cheaper.
https://www.schenckandcompany.com/wood/wood-hardness-chart/

I thought the interply adhesives in marine plywood were thermosetting. They would present if anything a harder confirming surface for the actual lubricating oil or grease to run against. Certainly I encourage anyone in a position to run non-critical tests on this to do so.

Technically a deflicted axle could be machined smaller (anth a brass made to match) as loads in typical 'preservation' service for equipment with plain bearings will likely be much smaller and operations much slower than original revenue service. Likewise it would be comparatively simple, if not cheap, to conduct proper metalspraying followed by machining/burnishing and perhaps a hard coating, But the moral of this whole thread is DON'T HAVE TO FIND OUT HOW TO REMEDIATE JOURNALS in the first place.

I think there may be a bit of over-engineering going on for a move of "several hundred feet" as stated in the original post.

Practically any wooden interface is going to suffice here.

There's a hydro plant not far from here whose generators are running on "Oak" Block Bearings that are water-cooled; they saw no need to do otherwise during a retro-fit some years ago; replaced the Oak Blocks with New Oak Blocks.
Probably use "Lapacho" in a pinch!

This plant is operated by a major utility.

Our hydro plant uses Lignum Vitae bearings, lubed by the water turning the turbine.

I think we're getting into the overthinking solutions more difficult that the project under consideration situation here. Use the best wood you have available, grains running horizontally, grease the crap out of everything, make the move slowly then take the wood bearings out and store them in a can of oil ready for the next short slow move.

Well, update time. I found out this car has already been 'test' moved about 20 feet back and forth. All the boxes, liners, and axles were in random places, some of the journal boxes were several inches too far away from the wheels, and some of the liners were half out and crooked. That amount of movement put about a 1/8 inch gouge in some of the liners. I improvised some 'bearings' out of plain 2x4 pine and that worked out pretty well. I'll start a new thread with details on how I cut them so maybe someone in the future can search that out and be prepared if they have to do this on short notice. For me, it was about a 6 hour job between fabricating the blocks and working the trucks back into 'square' so everything would line back up and fall into place, in 90+ degree heat. If I did it again I would cut them a bit different so they fit easier...I had to hammer some of the liners into place due to 2x4 is a bit thicker then ideal.

and you know there has to be a wedge between the top of the box and the top of the brass, it's actually an adapter, the box isn't supposed to rest directly on the brass. And the brass is babbitt lined, only the babbit contacts the journal.

A wedge will likely be found with all interchange equipment, when you are working on early and/or narrow gauge equipment, you find that not all journal bearings/boxes use wedges... some use brass bearings without babbit.

I have used a simple piece of 2x4 when moving a car a few hundred feet... I have used very carefully shaped oak blocks in place of bearings for long periods of time (but not for long distance or frequent operation.)

One issue with wood bearings... have the grain of the wood run across the axle, not with the axle to prevent the block from splitting.

Good point on the grain.

I’d wager that if the brasses are gone, so are the wedges so be careful.

It is not recommended to move a railcar that has had its journal brasses missing, even for a short distance. The journal brasses are essential for preventing damage to the axle and bearing. Without them, the axle could wear down quickly, leading to a derailment. Additionally, the bearings could overheat and seize, which could also cause a derailment, you can visit at https://en.tradebearings.com/list_29.html.

In a pinch, it may be possible to move a railcar a short distance without its journal brasses, but it should only be done as a last resort and with extreme caution. The railcar should be moved slowly and carefully, and it should be inspected for damage after it has been moved.

Was called to work the Operators position at Niles Jct. on the B&O one night as the regular man had marked off 'sick'. Ended up working there for about six weeks. The 'sick' operator and a buddy of his were arrested that night with a overloaded pickup truck load of journal brass they at stolen from cars being loaded at location known as Rock Cut. Brass get heavy when the numbers add up.

In the early 80's, Santa Fe stored a bunch of 40 foot box cars off the siding at Peabody, KS. Sometimes they would have an eastbound stop on the main, cut off, go in on those stored cars, air them up, couple up the crossings, and roll the cars back and forth a few car lengths, to keep then lubed. One time somebody had stolen most of the brass on the south side, and nobody knew, and it's not something a train crew would even look for, Of course it ruined all the journals.

Found this at an old wreck site.