When you mention the German invasion of Holland in May 1940, most people can't name any of the battles there. The best that anyone can remember is the bombing of Rotherham, or occasionally, for the geographically confused, Eben Emael. However during the first day of the German invasion the Dutch put up a fight giving the Germans a bloody nose. All of this despite the poor state of the Dutch defenders.
The Dutch forces were underfunded and largely ill equipped, with many weapons being obsolete. Plus I think their uniforms looked silly and were a horrific colour. Equally the Dutch defences had some serious problems. Constructed during peacetime they often had to take into consideration civilian property, so you might get a nicely sited bunker, with a house or a farm creating dead ground right in front of it. On the plus side the Dutch had a large number of north/south waterways and boggy ground. Equally the Germans didn't have as many tanks facing Holland, due to most being concentrated further south against the French. To make up for this deficiency the Germans planned to deploy a number of armoured trains against Holland.
Dutch Soldiers standing guard, literally, at the border with Germany.
At the Dutch town of Mill, the soldiers with their antique 84mm field guns were just receiving word that war had broken out. War had been declared only half an hour earlier when two trains chugged into view. The Dutch forces thought they were Dutch trains and let them pass unmolested, in reality the lead train was the small German armoured train, Panzerzug 1. It carried a small detachment of manpower. Behind it was a fully loaded troop train carrying an entire German infantry battalion.

The two trains passed the Dutch line without any serious opposition, and proceeded to move east further into Dutch territory. Near the town of Zeeland the armoured train had its airline hit forcing the column to halt for repairs while the Germans fought off the Dutch defenders.
You might be confused about how an armoured train could be knocked out by small arms fire, well put out of your mind the armoured trains from later in the war. These early German trains were little more than wagons protected with steel plate and a much more ramshackle affair than you are expecting, many were only armed with loopholes for riflemen, or a Maxim gun or two.
While Panzerzug 1 was being repaired the troop train attempted to contact its headquarters by radio to report the success of their penetration. However the short range radios were unable to reach the much delayed HQ. After the repairs had been made to Panzerzug 1 it was decided that she would return to the headquarters carrying a message. The armoured train was moved to a siding at the station of Zeeland, and placed behind the Troop train for its run back to friendly lines.
As it rattled along and approached the bridge at Mill a warning was yelled. The Dutch, far from being idle in the last hours had barricaded the train tracks. Panzerzug 1 locked its wheels in an effort to avoid the obstruction. It was somewhat successful. The front carriage was derailed as it hit the obstructions, and slid down the bank into the canal below.
Replica of the barricades the Dutch erected.
The few men on the train immediately dismounted and launched an assault against the two nearest Dutch bunkers, capturing both. Before they could push any further the Dutch soldiers began to fire and soon an intense firefight developed between the Dutch and German held bunkers.
Claimed to be Panzerzug 1, but when you compare to a latter picture which is also claimed to be Panzerzug 1 you'll notice some differences.
The battalion on the troop train dismounted and began a series of fights to try and break the line from the reverse. At one point they came across a battery of Dutch field guns, which were unmapped by the Germans. As they were attacking into the guns flank the gun crews had to be careful as they were firing over each others heads, this hardly improved the guns terrible rate of fire. The guns themselves dated from 1881. However with the shelling and the crew using their personal weapons they managed to drive off the attacking German company after an hour or so of fighting. They then shifted their fire to the disabled German train, forcing the Germans who had been using the armoured carriages as a base of fire against the the rest of the Dutch line to abandon their superior position. The battle finally ended the following day as German reinforcements swamped the area.
Dutch 84mm 8-Staal guns at Mill
Other German trains fared little better. For example Panzerzug 6 first came across a swing bridge. These are a form of bridge used to allow ships to pass, they rotate on a central pillar. The Dutch forces had opened the bridge forcing the Germans to dismount and recapture the control centre to close the bridge so that they could proceed. Further on they encountered another bridge that had been demolished. After constructing a replacement they met a third and final bridge. This too was swing bridge which the Dutch had opened. Then to prevent a repeat of what happened at the first swing bridge they borrowed some rolling stock from the Dutch railway and crashed that into the turning mechanism jamming the bridge in the open position. With this impassable barrier Panzerzug 6 retreated. Elsewhere it was largely the same story. Even when Brandenburgers tried to capture the bridges dressed in Dutch uniforms they failed.
Also claimed ot be Panzerzug 1, it at least gives you an idea of the nature of the Armoured trains in use.
After their disastrous showing in the Dutch campaign, the armoured trains were recalled to Germany. Slowly the concept changed to the heavily armoured and armed concept you're more familiar with, such as the BP-42 train which went on to see action in the invasion of Russia.

Image credits:
kennispuntmei1940.nl and www.worldwarphotos.info